[BUGFIX] FormEngine: Fix placeholder traversal for select type relations

When creating a new record that resolves the placeholder from a related
record via a select type field empty values get filtered out. Otherwise
an exception gets raised as 0 is not a positive integer.

Resolves: #70146
Releases: master
Change-Id: I2e49380afed40df160db7e911d3128e6664f33d0
Reviewed-on: https://review.typo3.org/44440


Reviewed-by: Sebastian Fischer <typo3@evoweb.de>
Tested-by: Sebastian Fischer <typo3@evoweb.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>