Fix TypeError failing CI (#108)

TypeError: SMW\Localizer\Localizer::asBCP47FormattedLanguageCode(): Argument #1 ($languageCode) must be of type string, null given, called in /var/www/html/extensions/SemanticInterlanguageLinks/src/Search/SearchResultModifier.php on line 148

Author: paladox

src/Search/SearchResultModifier.php

@@ -145,7 +145,9 @@ public function applyLanguageFilterToResultMatches( $request, &$titleMatches, &$
 			return false;
 		}
 
-		$languageCode = Localizer::asBCP47FormattedLanguageCode( $request->getVal( 'languagefilter' ) );
+		$languageCode = Localizer::asBCP47FormattedLanguageCode(
+			$request->getVal( 'languagefilter' ) ?? ''
+		);
 
 		if ( in_array( $languageCode, [ null, '', '-' ] ) ) {
 			return false;