Fix LanguageNegotiationContentEntity::getLangcode() errors thrown

Now that 🐛 Fix LanguageNegotiationMethodInterface::getLangcode() result docblock Fixed is fixed and there is no documentation-inconsistency aspect, this here issue can move forward. Setting it back to NR since my latest changes set the return values to NULL (as was originally) instead of FALSE. That way the change-set focuses only on the reported errors (see description).

Thanks @stefanos.petrakis for creating the separate issue and following this one through

This looks good to me

Committed and pushed 5738144ad9 to 10.1.x and 681c999fea to 10.0.x. Thanks!

Not backporting to 9.5.x because the test needs changing because it has property typehints and we still support PHP 7.3 there and this is a minor bug.

Automatically closed - issue fixed for 2 weeks with no activity.