fix(sdk): `.bytes()` inconsistent behavior between groth16 and plonk

[ratankaliani]

Motivation

Inconsistent behavior in bytes between mock PLONK and Groth16 proofs. This caused an issue in op-succinct.

In the future, especially for PR's from external folks, we should have simple tests added in the relevant file to test behavior. This prevents issues such as this which lead to inconsistent API behavior.

Reversion introduced by https://github.com/succinctlabs/sp1/pull/1685. Likely because base dev branch was not up to date.

Solution

Return an empty byte array from .bytes() if it's a mock Groth16 proof.

Add testing to confirm that .bytes() returns the correct output in the future.

Note: Did not test SP1Proof::Compressed because constructing an empty ReduceProof was difficult.

PR Checklist

• [X] Added Tests
• [X] Added Documentation

[github-actions[bot]]

SP1 Performance Test Results

Branch: ratan/return-empty-bytes-groth16 Commit: 64753139 Author: ratankaliani

program	cycles	execute (mHz)	core (kHZ)	compress (KHz)	time	success
fibonacci	11291	0.18	2.78	0.46	25s	✅
ssz-withdrawals	2757356	17.48	128.61	35.09	1m19s	✅
tendermint	12593597	6.71	267.42	98.68	2m9s	✅