[BUGFIX] Fix persisting new objects

While adding new records, extbase throws:
The given row must contain a value for "uid"

This is a follow up to introducing explicit saving
of modified domain objects, where the lines inserting new
objects was removed.

Fixes: #47546
Related: #47251
Releases: 6.2, 6.1
Change-Id: I8f3e9eef86e989c5092a1b5f312151e2bd78a790
Reviewed-on: https://review.typo3.org/20134
Reviewed-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring