Typing: fix type of func.coalesce when used with hybrid properties

Typing: fix type of func.coalesce when used with hybrid properties

Fixed typing issue where :class:`.coalesce` would not return the correct
return type when a nullable form of that argument were passed, even though
this function is meant to select the non-null entry among possibly null
arguments.  Pull request courtesy Yannick PÉROUX.

Closes: #12963
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12963
Pull-request-sha: 05d0d9784d4497fb3bfee540fbc51747c1767c90

Change-Id: Ife83a384ea57faf446c1fdb542df14627348f40f