**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.
for (const auto & elem : from_tuple->getElements())
            {
                const auto * type_array = checkAndGetDataType<DataTypeArray>(elem.get());
                if (!type_array)
                    throw Exception(ErrorCodes::TYPE_MISMATCH,
                        "CAST AS Map can only be performed from tuples of array. Got: {}", from_tuple->getName());
                from_kv_types.push_back(type_array->getNestedType());
            }
            return createTupleToMapWrapper(from_kv_types, to_kv_types);
        }
        else if (const auto * from_array = typeid_cast<const DataTypeArray *>(from_type_untyped.get()))
        {
            const auto * nested_tuple = typeid_cast<const DataTypeTuple *>(from_array->getNestedType().get());
            if (!nested_tuple || nested_tuple->getElements().size() != 2)
                throw Exception{"CAST AS Map from array requeires nested tuple of 2 elements.\n"
                    "Left type: " + from_array->getName() + ", right type: " + to_type->getName(), ErrorCodes::TYPE_MISMATCH};
            return createArrayToMapWrrapper(nested_tuple->getElements(), to_type->getKeyValueTypes());
        }
        else if (const auto * from_type = checkAndGetDataType<DataTypeMap>(from_type_untyped.get()))
        {
            return createMapToMapWrrapper(from_type->getKeyValueTypes(), to_type->getKeyValueTypes());
        }
        else
        {
            throw Exception{"Unsupported types to CAST AS Map\n"
                "Left type: " + from_type_untyped->getName() + ", right type: " + to_type->getName(), ErrorCodes::TYPE_MISMATCH};
        }
    }
    template <typename FieldType>
    WrapperType createEnumWrapper(const DataTypePtr & from_type, const DataTypeEnum<FieldType> * to_type) const