[BUGFIX] Allow attribute value "0" in EXT:form elements

If an attribute value of "0" is set in an EXT:form element (e.g.
TEXTLINE) currently the input element in the frontend is rendered with
an empty value "".

Do not check attribute values with empty() to allow "0" values.

Resolves: #31920
Releases: master, 6.2
Change-Id: I624c7da38c23429f68000e085dbd82c23fb7e834
Reviewed-on: http://review.typo3.org/35443
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>