[BUGFIX] Check if at least one table is selected for cleanup

Check if value to iterate over is set and is array. Without this
check the install tool breaks if no table is selected for cleanup
and displayErrors is set >= 1.

Resolves: #68030
Releases: master
Change-Id: I8af5c91107c70210001726cfb90e0e446f3e2450
Reviewed-on: http://review.typo3.org/41066


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>