[BUGFIX] Make ContentSlide work correctly if last element is empty

When using ContentSlide and the last content element in a column returns
an empty string, the slide continues up in the page list, even when
there are other content elements in the column.
Fix this by checking the correct value.

Change-Id: I6543cf2335197ae8510aa8d805d4d60a8d8a2320
Resolves: #51936
Releases: master, 6.2
Reviewed-on: http://review.typo3.org/32928
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Markus Klein <klein.t3@reelworx.at>