further fixes for _cleanup_mapped_str_annotation

further fixes for _cleanup_mapped_str_annotation

Fixed issues in type handling within the ``type_annotation_map`` feature
which prevented the use of unions, using either pep-604 or ``Union``
syntaxes under future annotations mode, which contained multiple generic
types as elements from being correctly resolvable.

also adds some further tests to assert that None added into the type
map for pep695, typing.NewType etc. sets up nullability on the column

Fixes: #12207
Change-Id: I4057694cf35868972db2942721049d79301b19c4
(cherry picked from commit fd3d17a30b15cc45ba18efaeb24ecc29b0ea1087)