[BUGFIX] Fix PHP warning with CTYPE=media

Check for array type before checking the variable with isset().
If the variable is a string the isset() will cause a PHP warning.

Resolves: #59544
Releases: master, 6.2
Change-Id: I766292a3dc0365a40acff5fd5f089f7ea78e0b70
Reviewed-on: http://review.typo3.org/33212
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>