[BUGFIX] Illegal string offset in EditDocumentController

If a content element is opened from the opendocs module, a warning is
shown because overrideVals is used without checking if it is an array.

Fixes: #46965
Releases: 6.1, 6.0, 4.7, 4.5
Change-Id: If25b21121708226594cfac6436278c297f32ddb9
Reviewed-on: https://review.typo3.org/19636
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn