hal-internal: fix failing test due to not initializing ringbuf.

embassy-hal-internal/src/atomic_ring_buffer.rs

@@ -478,8 +478,12 @@ mod tests {
 
     #[test]
     fn zero_len() {
+        let mut b = [0; 0];
+
         let rb = RingBuffer::new();
         unsafe {
+            rb.init(b.as_mut_ptr(), b.len());
+
             assert_eq!(rb.is_empty(), true);
             assert_eq!(rb.is_full(), true);