Upgrade PHPStan level to 5

[MAILPOET-1915]

lib/Util/Security.php

@@ -15,7 +15,7 @@ class Security {
   static function generateRandomString($length = 5) {
     // non-cryptographically strong random generator
     return substr(
-      md5(uniqid(mt_rand(), true)),
+      md5(uniqid((string)mt_rand(), true)),
       0,
       min(max(5, (int)$length), 32)
     );

lib/Util/Url.php

@@ -10,7 +10,7 @@ class Url {
 
     // Remove WPFunctions::get()->homeUrl() path from add_query_arg
     if (isset($home_url['path'])) {
-      $query_args = str_replace($home_url['path'], null, $query_args);
+      $query_args = str_replace($home_url['path'], '', $query_args);
     }
 
     return WPFunctions::get()->homeUrl($query_args);