Implicitly marking parameter $... as nullable is deprecated in PHP
8.4. The explicit nullable type must be used instead.
Bug: T376276
Change-Id: I251cb37401c37242f493816b6f70ab61a64a4c32
Most notably:
* Use the much more narrow UserIdentity interface where possible.
* Make array type hints in PHPDocs as specific as possible.
Change-Id: Id189da4028b7874909277881dcf6539169dd13b6