[BUGFIX] Value 0 in integer fields is not shown in List module anymore

A side-effect of #27936 is that the value 0 of integer fields is not
shown anymore in the List module. This error was also catched by the
failing Unit Test t3lib_befuncTest::getProcessedValueForZeroStringIsZero

Change-Id: I07da5d05c08a9066e8e9c8ad68d3f83ef6f7dbcb
Resolves: #28218
Relates: #27936
Releases: 4.6, 4.5, 4.4, 4.3
Reviewed-on: http://review.typo3.org/3369
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack