[BUGFIX] FormEngine: Fix IRRE handling in flexforms

This patchset solves multiple problems with handling IRRE within a
FlexForm:

 * creating new records no longer triggers an internal server error
   due to missing parent data
 * IRRE children are passed to the renderer so that they get displayed
   in the backend
 * child table is determined correctly within flexforms when toggling
   expand/collapse state.

Resolves: #70918
Resolves: #70859
Releases: master
Change-Id: I93abeab696fff356453f0a1c305a8cfeede7158c
Reviewed-on: https://review.typo3.org/44579


Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>