Fix clippy warning for `len_zero`

Summary

When using an AllocRingBuffer from the crate and performing a check with len() == 0, Clippy does not recognize it as an optimization opportunity and does not provide a warning or suggestion. This behavior is inconsistent compared to how Clippy handles standard arrays.

Example

use ringbuffer::{AllocRingBuffer, RingBuffer};

fn main() {
    let buffer: AllocRingBuffer<u32> = AllocRingBuffer::new(5);
    if buffer.len() == 0 {
        println!("Clippy should warn here.");
    }

    let array = [0, 10, 100];
    if array.len() == 0 {
        println!("Clippy does warn here.");
    }
}

To reproduce this, run cargo clippy or cargo +nightly clippy and observe that no warning is triggered for AllocRingBuffer, whereas it works correctly for standard arrays.