[BUGFIX] Empty class selector crashes Update image

Prevent the Update image dialog box to crash when trying to access
an empty class selector.

Resolves: #59938
Releases: 6.2, 6.3
Change-Id: Ida2c3dfb185064071fcdcda158f64e330562aa01
Reviewed-on: https://review.typo3.org/31350
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters