[BUGFIX] FormEngine creating section container in new record

If a records has not been persisted and a flex section container
is added, the form compiler needs the "NEW123" uid of parent to
prefix form fields correctly.

Change-Id: Iab3945efa93d2becf636c5cf4f8596bc696cd2e0
Resolves: #80825
Related: #80100
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/53215


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>