fix: allow delete from addr book when username is null (openemr#7806)

interface/usergroup/addrbook_edit.php

@@ -232,7 +232,7 @@ function typeSelect(a) {
 } elseif (!empty($_POST['form_delete'])) {
     if ($userid) {
        // Be careful not to delete internal users.
-        sqlStatement("DELETE FROM users WHERE id = ? AND username = ''", array($userid));
+        sqlStatement("DELETE FROM users WHERE id = ? AND (username = '' OR username IS NULL)", array($userid));
     }
 }