**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 NameToUInt16 { static constexpr auto name = "toUInt16"; };
struct NameToUInt32 { static constexpr auto name = "toUInt32"; };
struct NameToUInt64 { static constexpr auto name = "toUInt64"; };
struct NameToUInt128 { static constexpr auto name = "toUInt128"; };
struct NameToUInt256 { static constexpr auto name = "toUInt256"; };
struct NameToInt8 { static constexpr auto name = "toInt8"; };
struct NameToInt16 { static constexpr auto name = "toInt16"; };
struct NameToInt32 { static constexpr auto name = "toInt32"; };
struct NameToInt64 { static constexpr auto name = "toInt64"; };
struct NameToInt128 { static constexpr auto name = "toInt128"; };
struct NameToInt256 { static constexpr auto name = "toInt256"; };
struct NameToFloat32 { static constexpr auto name = "toFloat32"; };
struct NameToFloat64 { static constexpr auto name = "toFloat64"; };
struct NameToUUID { static constexpr auto name = "toUUID"; };
using FunctionToUInt8 = FunctionConvert<DataTypeUInt8, NameToUInt8, ToNumberMonotonicity<UInt8>>;
using FunctionToUInt16 = FunctionConvert<DataTypeUInt16, NameToUInt16, ToNumberMonotonicity<UInt16>>;
using FunctionToUInt32 = FunctionConvert<DataTypeUInt32, NameToUInt32, ToNumberMonotonicity<UInt32>>;
using FunctionToUInt64 = FunctionConvert<DataTypeUInt64, NameToUInt64, ToNumberMonotonicity<UInt64>>;
using FunctionToUInt128 = FunctionConvert<DataTypeUInt128, NameToUInt128, ToNumberMonotonicity<UInt128>>;
using FunctionToUInt256 = FunctionConvert<DataTypeUInt256, NameToUInt256, ToNumberMonotonicity<UInt256>>;
using FunctionToInt8 = FunctionConvert<DataTypeInt8, NameToInt8, ToNumberMonotonicity<Int8>>;
using FunctionToInt16 = FunctionConvert<DataTypeInt16, NameToInt16, ToNumberMonotonicity<Int16>>;
using FunctionToInt32 = FunctionConvert<DataTypeInt32, NameToInt32, ToNumberMonotonicity<Int32>>;
using FunctionToInt64 = FunctionConvert<DataTypeInt64, NameToInt64, ToNumberMonotonicity<Int64>>;
using FunctionToInt128 = FunctionConvert<DataTypeInt128, NameToInt128, ToNumberMonotonicity<Int128>>;
using FunctionToInt256 = FunctionConvert<DataTypeInt256, NameToInt256, ToNumberMonotonicity<Int256>>;
using FunctionToFloat32 = FunctionConvert<DataTypeFloat32, NameToFloat32, ToNumberMonotonicity<Float32>>;
using FunctionToFloat64 = FunctionConvert<DataTypeFloat64, NameToFloat64, ToNumberMonotonicity<Float64>>;
using FunctionToDate = FunctionConvert<DataTypeDate, NameToDate, ToDateMonotonicity>;
using FunctionToDate32 = FunctionConvert<DataTypeDate32, NameToDate32, ToDateMonotonicity>;