tidy: Use is_empty() instead of len tests

Fixes a clippy warning, and improves readability.

src/tools/tidy/src/deps.rs

@@ -239,7 +239,7 @@ pub fn check_whitelist(path: &Path, cargo: &Path, bad: &mut bool) {
         unapproved.append(&mut bad);
     }
 
-    if unapproved.len() > 0 {
+    if !unapproved.is_empty() {
         println!("Dependencies not on the whitelist:");
         for dep in unapproved {
             println!("* {}", dep.id_str());

src/tools/tidy/src/features.rs

@@ -133,7 +133,7 @@ pub fn check(path: &Path, bad: &mut bool, quiet: bool) {
                  name);
     }
 
-    if gate_untested.len() > 0 {
+    if !gate_untested.is_empty() {
         tidy_error!(bad, "Found {} features without a gate test.", gate_untested.len());
     }