Fix rendering issue for LDAP configuration provider transactions

Summary: Fixes T4290. At least one of the fields (`realname`) may have a list of items, and `strlen(array('first', 'last'))` produces the warning and stack trace in T4290.

Test Plan:
  - Edited `realname` from an array value to an array value.
  - Hit error.
  - Applied patch.
  - No more error.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4290

Differential Revision: https://secure.phabricator.com/D7905

src/applications/auth/provider/PhabricatorAuthProviderLDAP.php

@@ -361,7 +361,7 @@ final class PhabricatorAuthProviderLDAP
           $label);
       }
 
-      if (!strlen($old)) {
+      if ($old === null || $old === '') {
         return pht(
           '%s set the "%s" value to "%s".',
           $xaction->renderHandleLink($author_phid),