style: add None for empty return statements in server.py

[mrKazzila]

Summary

Simply added None to empty return to maintain consistency with other methods/functions that return None.
Example:

1. ruff-lsp/ruff_lsp/server.py

   Line 622 in bae2b24

   return None

2. ruff-lsp/ruff_lsp/server.py

   Line 733 in bae2b24

   return None

3. …

Also aligned with PEP 8 recommendations "Be consistent in return statements …".

Test Plan

N/A because both return and return None return None in the same way.

[charliermarsh]

Thanks!