[SIM907] Use Optional[Type] instead of Union[Type, None]

## Explanation

Sometimes people tend to write ``Union[Type, None]`` instead of ``Optional[Type]``.

## Example

```python
# Bad
def foo(a: Union[int, None]) -> Union[int, None]:
  return a

# Good
def foo(a: Optional[int]) -> Optional[int]:
  return a
```
