[BUGFIX] Prevent error when overlay is not available

[magicsunday]

This fixes:

Core: Error handler (BE): PHP Warning: Trying to access array offset on value of type null in /var/www/vendor/apache-solr-for-typo3/solr/Classes/ContentObject/Relation.php line 311

What this pr does

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How to test

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Fixes: #issue (Please create an related issue first and mark it as fixed here with your pr)

[dkd-kaehm]

@magicsunday
Thanks for fixing that issue please check following error
by https://github.com/TYPO3-Solr/ext-solr/actions/runs/12273510045/job/34244520075#step:10:78:

 ------ ----------------------------------------------------------------------------- 
  Line   Classes/ContentObject/Relation.php                                           
 ------ ----------------------------------------------------------------------------- 
  306    Method                                                                       
         ApacheSolrForTypo3\Solr\ContentObject\Relation::getFrontendOverlayService()  
         invoked with 1 parameter, 0 required.                                        
  313    Variable $overlayRelatedRecord might not be defined.                         
 ------ ----------------------------------------------------------------------------- 

[dkd-kaehm]

Currently fixed the PHPstan issue, but do not merge it.
Relation MUST NOT always fallback to default language, but accept null as well.
Also it must follow the TYPO3s language modies(fallback, strict, free).

[magicsunday]

I think this can be closed, The relevant issue is #4245, with the fix for v12 release, which should than included in the main branch either.

[dkd-kaehm]

I think this can be closed, The relevant issue is #4245, with the fix for v12 release, which should than included in the main branch either.

Nope, please do not close.
release-12.0.x and main branches live their own lives, each change on them must be ported via separate Pull-Requests.