Allow empty input haystacks in MD_STD::stri_contains()

src/MD_STD.php

@@ -177,7 +177,7 @@ public static function startsWithAny(string $haystack, array $needles):bool {
     /**
      * Function checking if a string contains another in a case-insensitive way.
      *
-     * @param non-empty-string $haystack String to check.
+     * @param string           $haystack String to check.
      * @param non-empty-string $needle   Potential start of $haystack.
      *
      * @return boolean