make clippy happy

src/main.rs

@@ -96,7 +96,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let config = spawn_blocking(move || std::fs::read_to_string(urls_filename)).await??;
     let mut urlmap = HashMap::new();
     for url_pair in config.split('\n') {
-        if url_pair.len() == 0 {
+        if url_pair.is_empty() {
             continue;
         }