pad: Don't drop LATENCY queries with default implementation

pad: Don't drop LATENCY queries with default implementation

If there is only one pad in the internal pads, when folding for
LATENCY queries it will just drop the response if it's not live.

This is maybe not the proper fix, but it will just accept the first
peer responses, and if there are any other pads, it will only take
them into account if the response is live.

This *should* properly handle the aggregation/folding behaviour of
multiple live peer responses, while at the same time handling the
simple one-pad-only-and-forward use-case

https://bugzilla.gnome.org/show_bug.cgi?id=766360