[BUGFIX] Fix return type of determineSiteScript

substr returns a boolean instead of an empty string. A typecast fixes
this problem (See http://php.net/manual/de/function.substr.php#90369)

Resolves: #87387
Releases: master, 9.5
Change-Id: I14aa49c18ae028dc27b94bfdbfc3dfb643c18746
Reviewed-on: https://review.typo3.org/59395


Tested-by: TYPO3com <noreply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Alexander Schnitzler <typo3@alexanderschnitzler.de>
Tested-by: Alexander Schnitzler <typo3@alexanderschnitzler.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>