[BUGFIX] Use early return in cObj::getImgResource

If ContentObjectRenderer::getImgResource is called without any
data, there is no need to execute all possibilities and lookup
files with an empty identifier.
For instance TextMenuContentObject::getBeforeAfter triggers
this at lot.

Resolves: #66028
Releases: master, 6.2
Change-Id: Iee7a3ce0f2956cd6ddae8c2e3d194f3cf2855c49
Reviewed-on: http://review.typo3.org/38162


Reviewed-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Tested-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>