[BUGFIX] Make getFirstTag() return empty string for input without tag

getFirstTag() didn't take into account that strpos() returns FALSE when
the needle isn't found.

Resolves: #69318
Releases: master
Change-Id: Ie530db9cbed92fa11199852cc0aec77024354814
Reviewed-on: http://review.typo3.org/42898


Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Tested-by: Alexander Opitz <opitz.alexander@googlemail.com>