Remove length comparison to zero

Clippy emits:

  warning: length comparison to zero

Remove length comparison to zero, use `!is_empty`.

src/util/psbt/mod.rs

@@ -860,7 +860,7 @@ mod tests {
             assert_eq!(redeem_script.to_p2sh(), expected_out);
 
             for output in psbt.outputs {
-                assert!(output.get_pairs().unwrap().len() > 0)
+                assert!(!output.get_pairs().unwrap().is_empty())
             }
         }