patch 9.1.0976: Vim9: missing return statement with throw

patch 9.1.0976: Vim9: missing return statement with throw

Problem:  Vim9: missing return statement with throw
          (atitcreate)
Solution: Treat a throw statement at the end of an if-else block as a
          return statement (Yegappan Lakshmanan)

fixes: #16312
closes: #16338

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>