[BUGFIX] Make fetchUserRecord callable without username

t3lib_userAuth::fetchUserRecord is not callable without a username.
Fix the $usernameClause to generate a correct SQL query.

Change-Id: I56b5f8bf8b326e880f157c3b4a5a15409305c3d9
Fixes: #39248
Releases: 6.1, 6.0, 4.7, 4.5
Reviewed-on: https://review.typo3.org/19776
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert