[BUGFIX] Log password attempt with empty password

Adds logging for login attempts where an empty password is
submitted.

Change-Id: I8733f388d3d919a7e7ff67fecd5704db20e78fe6
Resolves: #18176
Releases: 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/6904
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert
Reviewed-by: Mario Rimann
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter