Fix type of CASE expressions which include NULLs

Fix type of CASE expressions which include NULLs

Fixed issues in :func:`_sql.case` where the logic for determining the
type of the expression could result in :class:`.NullType` if the last
element in the "whens" had no type, or in other cases where the type
could resolve to ``None``.  The logic has been updated to scan all
given expressions so that the first non-null type is used, as well as
to always ensure a type is present.  Pull request courtesy David Evans.

updates to test suite to use modern fixture patterns by Mike

Fixes: #10843
Closes: #10847
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/10847
Pull-request-sha: 4fd5c39ab56de046e68c08f6c20cd1f7b2cb0e0d

Change-Id: I40f905ac336a8a42b617ff9473dbd9c22ac57505