[BUGFIX] Do not render empty help button

Currently, if EXT:cshmanual is not installed, an empty help button
gets rendered. This patch changes the return value of
BackendUtility::wrapInHelp() and checks for an empty string in
HelpButton.

Resolves: #71129
Releases: master
Change-Id: Ifec1b1b6f1289223fc2a453b88267fce4fb2c5a5
Reviewed-on: https://review.typo3.org/44355


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>