[BUGFIX] Fix PHP warning when writing to Backend user log

The writelog method for the BackendUserAuthentication object is now
checking if there is actually an uid available in the user array
before it tries to access it.

This prevents a PHP warning about an illegal string offset when writing
log messages in an authentication service.

Releases: 6.0, 6.1, 6.2
Resolves: #49328
Change-Id: I3eb028ab3ee00626328ebd2bbf2ad62a22773ac3