[BUGFIX] Check $prefix in ActionTask

Check if the prefix is not an empty string to avoid a warning thrown by
strpos().

Resolves: #86300
Related: #85477
Releases: master
Change-Id: I68a1fc83745594524c088600258c2925eccf3015
Reviewed-on: https://review.typo3.org/58320


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>