FlattenSentinel Type in Visitor Leave_X Calls Does Not Handle Empty Lines

For example, if you wanted to replace an if statement with a new block of code, including the prior empty lines, you can't return that type, because `EmptyLine` does not inherit from `BaseStatement`.

```
@mark_no_op
def leave_If(
self, original_node: "If", updated_node: "If"
) -> Union["BaseStatement", FlattenSentinel["BaseStatement"], RemovalSentinel]:
return updated_node
```

Given some code like:

```
# Comment
if some_statement:
print("Hello")
```

In order to correctly collapse the conditional, you must return a FlattenSentinel that includes both the EmptyLine that contains the comment (which is owned by `leading_lines` of the If node), as well as the node for the print statement. Today the return type doesn't allow this.