len_without_is_empty triggers for async len methods, but does not accept async is_empty methods

The following will trigger the `len_without_is_empty` error:

```rust
impl Foo {
    pub async fn len(&self) -> usize { ... }
    pub async fn is_empty(&self) -> bool { ... }
}
```

It probably shouldn't expect the exact signature `fn(&self) -> bool`.