[BUGFIX] Persist an empty unmodified objectstorage

If the propertymapper builds an empty objectstorage for a property or 
a property is set to a new objectstorage and the objectstorage property
has childobjects before, this objectstore needs to be persisted.

Change-Id: Ibcbd7cd6e1677e5321f0891e58f60bb5a49d529d
Releases: 6.1, 6.0, 4.7
Fixes: #13621
Fixes: #44110
Fixes: #42732
Reviewed-on: https://review.typo3.org/19964
Reviewed-by: Wouter Wolters
Reviewed-by: Georg Ringer
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Tested-by: Wouter Wolters
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs