fix(noise): return immediately when reading into an empty buffer

[modernzju]

Description

This updates libp2p-noise's AsyncRead implementation to return Poll::Ready(Ok(0)) immediately when poll_read is called with an empty destination buffer.

Previously, if no decrypted data was already buffered, reading into an empty buffer could still poll the underlying framed stream and return Pending. Returning immediately matches the expected AsyncRead behavior for zero-length reads and avoids unnecessary waiting.

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Attestation (required):

• I have read every line of this diff, understand what it does, and can explain it in review.

Notes & open questions

This does not change the Noise protocol framing or any public API.
It only handles the zero-length read case before polling the underlying stream.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[jxs]

Hi, and thanks for this.
Looking into the code I didn't see any calls to Output::poll_read with empty buffers, making this change redundant. Can you elaborate on why do you require this and fill out the PR form please?