Only log the propagating transactions when they are not empty (#5424)

This can make the log cleaner, especially when you specify `--log
sync=debug`.

prdoc/pr_5424.prdoc

+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Suppress the log output for transaction propagation when no transactions are present
+
+doc:
+  - audience: Node Dev
+    description: |
+      Previously, the log message `Propagating transactions` would always be printed, even when there were no transactions to propagate. This patch optimizes the logging by returning early when no transactions are present, resulting in cleaner and more relevant log output.
+
+crates:
+- name: sc-network-transactions
+  bump: none

substrate/client/network/transactions/src/lib.rs

@@ -522,8 +522,14 @@ where
 			return
 		}
 
-		debug!(target: LOG_TARGET, "Propagating transactions");
 		let transactions = self.transaction_pool.transactions();
+
+		if transactions.is_empty() {
+			return
+		}
+
+		debug!(target: LOG_TARGET, "Propagating transactions");
+
 		let propagated_to = self.do_propagate_transactions(&transactions);
 		self.transaction_pool.on_broadcasted(propagated_to);
 	}