Minor deprecation warning fix in Zend_Ldap. (#1013)

This fixes a minor deprecation warning caused by Zend_Ldap attempting to str_replace() on null, for plugins adding LDAP support to Omeka Classic via the Zend libraries.

application/libraries/Zend/Ldap.php

@@ -811,7 +811,7 @@ public function bind($username = null, $password = null)
 
         // Security check: remove null bytes in password
         // @see https://net.educause.edu/ir/library/pdf/csd4875.pdf
-        $password = str_replace("\0", '', $password);
+        $password = str_replace("\0", '', (string) $password);
 
         if ($username === null) {
             $username = $this->_getUsername();