In calls to mb_ functions, silently transform arg into string

In PHP8, a number of functions who were accepting null arguments will only accept
string ones.

In the polyfill, mb_* functions are declared with a trict type checking of "string".

Therefore, we deprecate the use of non string arguments, so that it won't break when either using the polyfill,
or future php8 versions.

Helper/Helper.php

@@ -47,6 +47,8 @@ public function getHelperSet()
      */
     public static function strlen($string)
     {
+        $string = (string) $string;
+
         if (false === $encoding = mb_detect_encoding($string, null, true)) {
             return \strlen($string);
         }
@@ -65,6 +67,8 @@ public static function strlen($string)
      */
     public static function substr($string, $from, $length = null)
     {
+        $string = (string) $string;
+
         if (false === $encoding = mb_detect_encoding($string, null, true)) {
             return substr($string, $from, $length);
         }