**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.
{
        if (from < 0)
            return 0;
        return from;
    }
};
/** Special case of converting Int8, Int16, (U)Int32 or (U)Int64 (and also, for convenience,
  * Float32, Float64) to Date. If the number is negative, saturate it to unix epoch time. If the
  * number is less than 65536, then it is treated as DayNum, and if it's greater or equals to 65536,
  * then treated as unix timestamp. If the number exceeds UInt32, saturate to MAX_UINT32 then as DayNum.
  * It's a bit illogical, as we actually have two functions in one.
  * But allows to support frequent case,
  *  when user write toDate(UInt32), expecting conversion of unix timestamp to Date.
  *  (otherwise such usage would be frequent mistake).
  */
template <typename Name> struct ConvertImpl<DataTypeUInt32, DataTypeDate, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeUInt32, DataTypeDate, ToDateTransform32Or64<UInt32, UInt16>> {};
template <typename Name> struct ConvertImpl<DataTypeUInt64, DataTypeDate, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeUInt64, DataTypeDate, ToDateTransform32Or64<UInt64, UInt16>> {};
template <typename Name> struct ConvertImpl<DataTypeInt8, DataTypeDate, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeInt8, DataTypeDate, ToDateTransform8Or16Signed<Int8, UInt16>> {};
template <typename Name> struct ConvertImpl<DataTypeInt16, DataTypeDate, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeInt16, DataTypeDate, ToDateTransform8Or16Signed<Int16, UInt16>> {};
template <typename Name> struct ConvertImpl<DataTypeInt32, DataTypeDate, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeInt32, DataTypeDate, ToDateTransform32Or64Signed<Int32, UInt16>> {};
template <typename Name> struct ConvertImpl<DataTypeInt64, DataTypeDate, Name, ConvertDefaultBehaviorTag>