[BUGFIX] Allow removal of search term in Page / List module

It wasn't possible to remove the search term used in the search
in the Page and List module below the docheader, because it was
checked for an empty submit button. The checks are removed now.

Resolves: #66552
Releases: master
Change-Id: I7113f5a00903326f9d12ee06b3510a376ff0f772
Reviewed-on: http://review.typo3.org/39743


Reviewed-by: Philipp Gampe <philipp.gampe@typo3.org>
Tested-by: Philipp Gampe <philipp.gampe@typo3.org>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>