BUGFIX] strftime expects parameter 2 to be long, string given

Any zero length string value is replaced with the current timestamp.
(Just like the default value for the second parameter of
strtime/gmstrftime.)

Change-Id: I7cc43a09f75d21ab8db84c0191fa0c1c5d67ddb0
Fixes: #38717
Releases: 6.0, 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/12713
Reviewed-by: Stefan Neufeind
Reviewed-by: Marcus Schwemer
Tested-by: Marcus Schwemer
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert