fix(swarm): poll handler after injecting LocalProtocolsChange event

Previously, we would not call the handler upon injecting `ConnectionEvent::LocalProtocolsChange`. This would prevent protocols from being able to react to this change and e.g. issue events or open streams.

Pull-Request: #3979.

swarm/src/connection.rs

@@ -425,12 +425,17 @@ where
             }
 
             let new_protocols = gather_supported_protocols(handler);
+            let changes = ProtocolsChange::from_full_sets(supported_protocols, &new_protocols);
 
-            for change in ProtocolsChange::from_full_sets(supported_protocols, &new_protocols) {
-                handler.on_connection_event(ConnectionEvent::LocalProtocolsChange(change));
-            }
+            if !changes.is_empty() {
+                for change in changes {
+                    handler.on_connection_event(ConnectionEvent::LocalProtocolsChange(change));
+                }
 
-            *supported_protocols = new_protocols;
+                *supported_protocols = new_protocols;
+
+                continue; // Go back to the top, handler can potentially make progress again.
+            }
 
             return Poll::Pending; // Nothing can make progress, return `Pending`.
         }