[FOLLOWUP][BUGFIX] Prevent wrong ref_index for fields with soft references

Prevent problems with NULL values in reference fields.

Releases: master, 6.2
Resolves: #60226
Change-Id: I168e508ca9d47a3047a7b11bed0c315186ff2e2a
Reviewed-on: http://review.typo3.org/37511


Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Mateusz Wojtuła <matw88@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>