Apply type correction to `TFSwinModelOutput`
# What does this PR do?

## Problem & Fix

This PR fixes small type error on `TFSwinModelOutput`.

`TFSwinModelOutput`'s `pooler_output` is actually optional, which can be disabled by using `add_pooling_layer=False` option on `TFSwinModel` or `TFSwinMainLayer`. This fix applies `Optional` type on `pooler_output` to fix this error.

## Review
This PR is related with Swin Transformer.
R: @amyeroberts, @NielsRogge

_The documentation is not available anymore as the PR was closed or merged._