Issue #3321942 Fix warning when static language is not existing

src/Services/StringPluralFormula.php

@@ -43,7 +43,7 @@ class StringPluralFormula implements StringPluralFormulaInterface {
   public function getDefaultPluralFormula($lang_code) {
     $default = 'n==1 ? 0 : 1';
     $static_lang_rules = $this->getStaticLanguageRules();
-    return $static_lang_rules[$lang_code] ?: $default;
+    return isset($static_lang_rules[$lang_code]) ? $static_lang_rules[$lang_code] : $default;
   }
 
   /**