Remove unnecessary Db::escape_string on empty string literal

Use a simple '' instead of Db::escape_string("") for the empty
description value in the INSERT statement.

Author: hxdimpf

okapi/services/caches/save_user_coords/WebService.php

@@ -92,7 +92,7 @@ private static function update_coordinates($cache_id, $user_id, $latitude, $long
                         '".Db::escape_string($longitude)."',
                         '".Db::escape_string($cache_id)."',
                         '".Db::escape_string($user_id)."',
-                        '".Db::escape_string("")."'
+                        ''
                     )
                 ");
             } else {