[BUGFIX] Use count on storage after initialization of LazyObjectStorage

The count in LazyObjectStorage for relation type HAS_MANY has
to use the count on storage items after initialization. Otherwise
the wrong count is returned for a not persisted storage.

Resolves: #54724
Releases: 6.2, 6.1, 6.0
Change-Id: I817ce86dde11b175e6a5765a8f6518770ba75f28
Reviewed-on: https://review.typo3.org/27443
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters