fix VBF_Get_Filter_List for the no filter case

If vbf_default_filter_list() returned without anything written to
the vsb (for no body or length 0), we would return as the filter
list whatever existed at ws->f + 1 prior to calling VBF_Get_Filter_List
(for example the string logged with std.log()).

Consequently, backend requests would fail with

	FetchError     Filter '...' not found

bin/varnishd/cache/cache_fetch.c

@@ -580,8 +580,12 @@ VBF_Get_Filter_List(struct busyobj *bo)
 		WS_MarkOverflow(bo->ws);
 		return (NULL);
 	}
+	if (VSB_len(vsb)) {
 		WS_Release(bo->ws, VSB_len(vsb) + 1);
 		return (VSB_data(vsb) + 1);
+	}
+	WS_Release(bo->ws, 0);
+	return ("");
 }
 
 static enum fetch_step