**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.
"CAST AS Array can only be performed between same-dimensional Array or String types");
        }
        DataTypePtr from_nested_type = from_type->getNestedType();
        
        bool from_empty_array = isNothing(from_nested_type);
        if (from_type->getNumberOfDimensions() != to_type.getNumberOfDimensions() && !from_empty_array)
            throw Exception(ErrorCodes::TYPE_MISMATCH,
                "CAST AS Array can only be performed between same-dimensional array types");
        const DataTypePtr & to_nested_type = to_type.getNestedType();
        
        const auto nested_function = prepareUnpackDictionaries(from_nested_type, to_nested_type);
        return [nested_function, from_nested_type, to_nested_type](
                ColumnsWithTypeAndName & arguments, const DataTypePtr &, const ColumnNullable * nullable_source, size_t ) -> ColumnPtr
        {
            const auto & array_arg = arguments.front();
            if (const ColumnArray * col_array = checkAndGetColumn<ColumnArray>(array_arg.column.get()))
            {
                
                ColumnsWithTypeAndName nested_columns{{ col_array->getDataPtr(), from_nested_type, "" }};
                
                auto result_column = nested_function(nested_columns, to_nested_type, nullable_source, nested_columns.front().column->size());
                
                return ColumnArray::create(result_column, col_array->getOffsetsPtr());
            }
            else
                throw Exception{"Illegal column " + array_arg.column->getName() + " for function CAST AS Array", ErrorCodes::LOGICAL_ERROR};
        };
    }
    using ElementWrappers = std::vector<WrapperType>;
    ElementWrappers getElementWrappers(const DataTypes & from_element_types, const DataTypes & to_element_types) const