ITS#10076 suffixmassage in back-asyncmeta does not handle empty remote suffix correctly

Unlike other proxies, asyncmeta does not currently support the rewrite engine. It has a simple suffix-massage functionality, which seemed to handle incorrectly a remote suffix of zero length - an extra separator was left at the end. Result entries were also processed incorrectly, a dn separator was missing. The fix is very simple and handles only this case, perhaps we should try to re-enable the rewrite engine in the future.