[BUGFIX] Fix access to empty ObjectStorage with numeric key

This prevents a warning error triggered by spl_object_hash being called
with an integer/string and can be caused by Fluid like this:

{object.relations.0}

Resolves: #62553
Releases: master, 6.2
Change-Id: Ie7d2a249e17f719142482781c2a96d093ff5f94c
Reviewed-on: http://review.typo3.org/33615
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>