[BUGFIX] Fix flexform inline record editing with empty uc

When the uc of a backend user is empty a unserialize error
will be shown. Fix this by checking the uc['inlineView']
correctly.

Resolves: #76561
Releases: master, 7.6
Change-Id: Ic7ad77333312b570bb29f46a908838085f020f50
Reviewed-on: https://review.typo3.org/48540


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>