[BUGFIX] Fix parseUserFuncArguments to respect 0 as argument

"0" must be an valid "last argument" value in the parseUserFuncArguments
method.

The implementation of the argument iteration must strictly check if
the arguments string is empty.

Change-Id: I8cee20a48030ea9163f55723d9027f39c43e5e1c
Resolves: #68132
Related: #61256
Releases: master, 6.2
Reviewed-on: http://review.typo3.org/41218


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>