Enable RET502: Forbid empty return when function returns Optional

## Rule

**RET502**: `implicit-return-value`

## Purpose

Forbid implicit `return` (empty return) in functions that return Optional values. Forces explicit `return None` for clarity.

## Example

```python
# Bad
def get_user(id: int) -> Optional[User]:
if not valid:
return # RET502 - implicit None
return find_user(id)

# Good
def get_user(id: int) -> Optional[User]:
if not valid:
return None # explicit
return find_user(id)
```

## Auto-fix

✅ Available

## Notes

- Improves code readability by making None returns explicit
- Helps distinguish between "forgot to return" and "intentionally returning None"

JIRA Issue: BA-4022