**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.
: DateTimeTransformImpl<DataTypeDateTime64, DataTypeDateTime, TransformDateTime64<ToDateTimeImpl>> {};
struct ToDateTime64Transform
{
    static constexpr auto name = "toDateTime64";
    const DateTime64::NativeType scale_multiplier = 1;
    ToDateTime64Transform(UInt32 scale = 0)
        : scale_multiplier(DecimalUtils::scaleMultiplier<DateTime64::NativeType>(scale))
    {}
    inline DateTime64::NativeType execute(UInt16 d, const DateLUTImpl & time_zone) const
    {
        const auto dt = ToDateTimeImpl::execute(d, time_zone);
        return execute(dt, time_zone);
    }
    inline DateTime64::NativeType execute(UInt32 dt, const DateLUTImpl & ) const
    {
        return DecimalUtils::decimalFromComponentsWithMultiplier<DateTime64>(dt, 0, scale_multiplier);
    }
};
/** Conversion of Date or DateTime to DateTime64: add zero sub-second part.
  */
template <typename Name> struct ConvertImpl<DataTypeDate, DataTypeDateTime64, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeDate, DataTypeDateTime64, ToDateTime64Transform> {};
template <typename Name> struct ConvertImpl<DataTypeDateTime, DataTypeDateTime64, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeDateTime, DataTypeDateTime64, ToDateTime64Transform> {};
/** Transformation of numbers, dates, datetimes to strings: through formatting.
  */
template <typename DataType>
struct FormatImpl
{
    static void execute(const typename DataType::FieldType x, WriteBuffer & wb, const DataType *, const DateLUTImpl *)