webrtcsrc: change the producer-id type for request-encoded-filter

With https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1477
the producer id used while emitting the request-encoded-filter
can be a None if the msid of the webrtcbin's pad is None.
This might not affect the signal handler written in C but
can panic in an existing Rust application with signal
handler which can only handle valid String type as its param
for the producer id.

So change the param type to Option<String> in the signal builder
for request-encoded-fiter signal

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1528>

net/webrtc/src/webrtcsrc/imp.rs

@@ -240,7 +240,7 @@ impl ObjectImpl for BaseWebRTCSrc {
                  */
                 glib::subclass::Signal::builder("request-encoded-filter")
                     .param_types([
-                        String::static_type(),
+                        Option::<String>::static_type(),
                         String::static_type(),
                         Option::<gst::Caps>::static_type(),
                     ])