livesync: Actually assume zero upstream latency when query fails

The code said "assuming zero" but left latency at None instead of
Some(0), failing to unwrap the value later.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1161>

utils/livesync/src/livesync/imp.rs

@@ -814,6 +814,7 @@ impl LiveSync {
                     imp: self,
                     "Can't query upstream latency -- assuming zero"
                 );
+                state.upstream_latency = Some(gst::ClockTime::ZERO);
             }
         }