[CI] Fixed phpstan comparison null to string issue

[ViniTou]

🎫 Issue	IBX-XXXXX

Description:

Given $availableLanguages returns only string[] -

Assert::assertContains($language, $availableLanguages);

make sure that $language is not null at this point. It could be dropped - but this way we keep actual intent when $language is null.

For QA:

Documentation:

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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