[BUGFIX] Property Mapper chokes on already-converted objects

Fixes also PropertyMappers chokes on already converted Objects if the
targetType is a compositeType like Collection<Entity>.
Backport from Flow issues #39445, #40661 and #36988

Change-Id: I8622dd9ef15a5e8e4aa98adc4e5b0def4a913ce1
Fixes: #44214
Fixes: #44215
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/18172
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Reviewed-by: Marc Neuhaus
Reviewed-by: Markus Günther
Tested-by: Markus Günther