Use ! is_empty instead of len > 0

As directed by clippy use `!a.is_empty()` instead of `a.len() > 0`.

testutils/src/lib.rs

@@ -349,7 +349,7 @@ impl TestClient {
 
     pub fn receive(&mut self, meta_tx: TestIncomingTx) -> Txid {
         assert!(
-            meta_tx.output.len() > 0,
+            !meta_tx.output.is_empty(),
             "can't create a transaction with no outputs"
         );