Fix for php 8.0 (#687)

* Fix PHP 8.0
$parts must be an array, because the argument for count() must be countable, mb_split() returns an array or a boolean.

libs/plugins/shared.mb_str_replace.php

@@ -44,7 +44,7 @@ function smarty_mb_str_replace($search, $replace, $subject, &$count = 0)
                 }
             }
         } else {
-            $parts = mb_split(preg_quote($search), $subject);
+            $parts = mb_split(preg_quote($search), $subject) ?: array();
             $count = count($parts) - 1;
             $subject = implode($replace, $parts);
         }