make getUserData() fail safely on non-string input

When removing multiple users at once, plugins listening on the
AUTH_USER_CHANGE event may pass the array of user names (params[0]
for 'delete') to

make getUserData() fail safely on non-string input

When removing multiple users at once, plugins listening on the
AUTH_USER_CHANGE event may pass the array of user names (params[0]
for 'delete') to getUserData(), which crashed authplain with an
"Array to string conversion" / illegal offset error.

Guard the bundled auth backends so a non-string user fails safely
by returning false, and document that AUTH_USER_CHANGE params[0] is
a string for create/modify but a string[] for delete.

Fixes #4567

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add needed type hints for phpunit8

This will break a lot of plugin tests, but can't be avoided

authplain: Add tests for group retrieval