allow len without is_empty

src/mania/mod.rs

@@ -123,6 +123,7 @@ pub struct ManiaStrains {
 impl ManiaStrains {
     /// Returns the number of strain peaks per skill.
     #[inline]
+    #[allow(clippy::len_without_is_empty)]
     pub fn len(&self) -> usize {
         self.strains.len()
     }