LanguageNegotiationContentEntity::getLangcode causes deprecations on PHP 8.5

Problem/Motivation

On PHP 8.5 the \Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationContentEntity::getLangcode() method results in deprecations when $request->query->get(static::QUERY_PARAMETER); return NULL - i.e. the query parameter is not set.

Steps to reproduce

Run the unit tests on PHP 8.5 - \Drupal\Tests\language\Unit\Plugin\LanguageNegotiation\LanguageNegotiationContentEntityTest with produce a deprecation.

Proposed resolution

Fix code to not produce a deprecation.

Remaining tasks

User interface changes

None

Introduced terminology

N/a

API changes

None

Data model changes

None

Release notes snippet

N/a