rtpgccbwe: Don't process empty lists

The structure parsing could result in an empty vector. Don't do any processing
since the loss code assumes it's non-empty for average estimates which would
result in weird/invalid results.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1181>

net/rtp/src/gcc/imp.rs

@@ -1184,6 +1184,8 @@ impl ObjectSubclass for BandwidthEstimator {
                                     })
                                     .collect::<Vec<Packet>>();
 
+                                // The list of packets could be empty once parsed
+                                if !packets.is_empty() {
                                     let bitrate_changed = {
                                         let mut state = this.state.lock().unwrap();
 
@@ -1200,6 +1202,7 @@ impl ObjectSubclass for BandwidthEstimator {
                                     }
                                 }
                             }
+                        }
 
                         gst::Pad::event_default(pad, parent, event)
                     },