[TASK] Strict string value handling in system extension form

Misconfigurations in e.g. TypoScript lead to cased "Array" strings.

Change-Id: I3e161b855d7628d2b2bfc10df9dab3e9ef1d5356
Resolves: #30086
Releases: 4.6
Reviewed-on: http://review.typo3.org/5109
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader