grandpa: always create and send justification if there are any subscribers (#6935)

* grandpa: use bytes type for justification rpc notification

* grandpa: always create justification if there are rpc subscribers

* grandpa: wording

* grandpa: replace notify_justification macro with function

* grandpa: prefer Option<&T> over &Option<T>

substrate/client/finality-grandpa/rpc/src/lib.rs

@@ -406,7 +406,7 @@ mod tests {
 
 		// Notify with a header and justification
 		let justification = create_justification();
-		let _ = justification_sender.notify(justification.clone()).unwrap();
+		justification_sender.notify(|| Ok(justification.clone())).unwrap();
 
 		// Inspect what we received
 		let recv = receiver.take(1).wait().flatten().collect::<Vec<_>>();
@@ -418,7 +418,7 @@ mod tests {
 
 		let recv_sub_id: String =
 			serde_json::from_value(json_map["subscription"].take()).unwrap();
-		let recv_justification: Vec<u8> =
+		let recv_justification: sp_core::Bytes =
 			serde_json::from_value(json_map["result"].take()).unwrap();
 		let recv_justification: GrandpaJustification<Block> =
 			Decode::decode(&mut &recv_justification[..]).unwrap();

substrate/client/finality-grandpa/rpc/src/notification.rs

@@ -23,10 +23,10 @@ use sc_finality_grandpa::GrandpaJustification;
 
 /// An encoded justification proving that the given header has been finalized
 #[derive(Clone, Serialize, Deserialize)]
-pub struct JustificationNotification(Vec<u8>);
+pub struct JustificationNotification(sp_core::Bytes);
 
 impl<Block: BlockT> From<GrandpaJustification<Block>> for JustificationNotification {
 	fn from(notification: GrandpaJustification<Block>) -> Self {
-		JustificationNotification(notification.encode())
+		JustificationNotification(notification.encode().into())
 	}
 }