Handle data callbacks of size zero

The client side does not allow CallbackData::Req of size zero. This
handles it on the server side by returning early.

The background for this is https://bugzilla.mozilla.org/show_bug.cgi?id=1907367
where something is causing data callbacks of size zero and tripping an
assertion, but we have not reproduced and an audit is not conclusive as
to where the root cause is.

server/src/server.rs

@@ -261,6 +261,11 @@ impl ServerStreamCallbacks {
             return cubeb::ffi::CUBEB_ERROR.try_into().unwrap();
         }
 
+        if nframes == 0 {
+            // Optimization: skip the RPC call when there are no frames.
+            return 0;
+        }
+
         let r = self.data_callback_rpc.call(CallbackReq::Data {
             nframes,
             input_frame_size: self.input_frame_size as usize,