[BUGFIX] Handle l10n_parent if field is no select-type

The value of l10n_parent is not necessarily an array.
It may as well be a string from a group-type or simple
input.
Adjust the condition for overlay detection accordingly.

Resolves: #77301
Releases: master, 7.6
Change-Id: I9119dfc0a55d623f037f15ddf1e11f132cf38c0b
Reviewed-on: https://review.typo3.org/49261


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>