**Describe the bug**

A combination of `use_join_nulls=1`, a `Nullable(enum)` column, `OUTER JOIN`, and `toString()` may throw an exception while generating a result.

**Does it reproduce on recent release?**

This happens on 21.6.3 stable.

**How to reproduce**

With the following setup:

```sql
CREATE TABLE a (
  key  String
) ENGINE=MergeTree ORDER BY key;

CREATE TABLE b (
  key  String,
  data Nullable(Enum16('a'=1, 'b'=2))
) ENGINE=MergeTree ORDER BY key;

INSERT INTO a VALUES('x');
INSERT INTO a VALUES('y');

INSERT INTO b VALUES('x', 'a');
```

This query appears to work correctly:
```
SELECT
	key, data
FROM a
ANY LEFT OUTER JOIN b USING (key)
FORMAT JSONCompact
SETTINGS join_use_nulls = 1;
```

```
{
        "meta":
        [
                {
                        "name": "key",
                        "type": "String"
                },
                {
                        "name": "data",
                        "type": "Nullable(Enum16('a' = 1, 'b' = 2))"
                }
        ],
        "data":
        [
                ["y", null],                                                                                                                                                                                                
                ["x", "a"]                                                                                                                                                                                                     
        ],
        "rows": 2,
        "statistics":
        {
                "elapsed": 0.002955967,
                "rows_read": 3,
                "bytes_read": 33
        }
}
```

But it throws an exception when `toString()` is applied to the joined column:

```sql
SELECT
	key, toString(data)
FROM a
LEFT OUTER JOIN b USING (key)
FORMAT JSONCompact
SETTINGS join_use_nulls = 1;
```

```
{
        "meta":
        [
                {
                        "name": "key",
                        "type": "String"
                },
                {
                        "name": "toString(data)",
                        "type": "Nullable(String)"
                }
        ],

        "data":
        [
                ["x", "a"]                                                                                                                                                                                                      Progress: 0.00 rows, 0.00 B (0.00 rows/s., 0.00 B/s.)
1 rows in set. Elapsed: 0.003 sec.

Received exception from server (version 21.6.3):
Code: 36. DB::Exception: Received from localhost:9000. DB::Exception: Unexpected value 0 in enum: while executing 'FUNCTION toString(data :: 1) -> toString(data) Nullable(String) : 2'.
```

**Expected behavior**

It produces a string `NULL` without throwing an exception.
struct NameToDecimal256OrZero { static constexpr auto name = "toDecimal256OrZero"; };
struct NameToUUIDOrZero { static constexpr auto name = "toUUIDOrZero"; };
using FunctionToUInt8OrZero = FunctionConvertFromString<DataTypeUInt8, NameToUInt8OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToUInt16OrZero = FunctionConvertFromString<DataTypeUInt16, NameToUInt16OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToUInt32OrZero = FunctionConvertFromString<DataTypeUInt32, NameToUInt32OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToUInt64OrZero = FunctionConvertFromString<DataTypeUInt64, NameToUInt64OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToUInt128OrZero = FunctionConvertFromString<DataTypeUInt128, NameToUInt128OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToUInt256OrZero = FunctionConvertFromString<DataTypeUInt256, NameToUInt256OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToInt8OrZero = FunctionConvertFromString<DataTypeInt8, NameToInt8OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToInt16OrZero = FunctionConvertFromString<DataTypeInt16, NameToInt16OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToInt32OrZero = FunctionConvertFromString<DataTypeInt32, NameToInt32OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToInt64OrZero = FunctionConvertFromString<DataTypeInt64, NameToInt64OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToInt128OrZero = FunctionConvertFromString<DataTypeInt128, NameToInt128OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToInt256OrZero = FunctionConvertFromString<DataTypeInt256, NameToInt256OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToFloat32OrZero = FunctionConvertFromString<DataTypeFloat32, NameToFloat32OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToFloat64OrZero = FunctionConvertFromString<DataTypeFloat64, NameToFloat64OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToDateOrZero = FunctionConvertFromString<DataTypeDate, NameToDateOrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToDate32OrZero = FunctionConvertFromString<DataTypeDate32, NameToDate32OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToDateTimeOrZero = FunctionConvertFromString<DataTypeDateTime, NameToDateTimeOrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToDateTime64OrZero = FunctionConvertFromString<DataTypeDateTime64, NameToDateTime64OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToDecimal32OrZero = FunctionConvertFromString<DataTypeDecimal<Decimal32>, NameToDecimal32OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToDecimal64OrZero = FunctionConvertFromString<DataTypeDecimal<Decimal64>, NameToDecimal64OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToDecimal128OrZero = FunctionConvertFromString<DataTypeDecimal<Decimal128>, NameToDecimal128OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToDecimal256OrZero = FunctionConvertFromString<DataTypeDecimal<Decimal256>, NameToDecimal256OrZero, ConvertFromStringExceptionMode::Zero>;
using FunctionToUUIDOrZero = FunctionConvertFromString<DataTypeUUID, NameToUUIDOrZero, ConvertFromStringExceptionMode::Zero>;
struct NameToUInt8OrNull { static constexpr auto name = "toUInt8OrNull"; };
struct NameToUInt16OrNull { static constexpr auto name = "toUInt16OrNull"; };
struct NameToUInt32OrNull { static constexpr auto name = "toUInt32OrNull"; };
struct NameToUInt64OrNull { static constexpr auto name = "toUInt64OrNull"; };
struct NameToUInt128OrNull { static constexpr auto name = "toUInt128OrNull"; };