[BUGFIX] PHP warning and invalid syslog entry for local users

Local backend users authenticating are generating a PHP warning
while writing the faulty syslog entry:

    Login-attempt from <IP> (), username '<username>' not found!!

Fixes: #60982
Change-Id: Ic40f7b320cc02b57440491c5fc45686be21619f7
Reviewed-on: http://review.typo3.org/32160
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>