Shplonk prover handles empty polynomials

[federicobarbacovi]

The Shplonk prover raises an error if it receives empty polynomials as it tries to access the zero-th coefficient of the polynomial.

[federicobarbacovi]

Audit of Shplonk is complete, this is not worth it. Also, we always ensure our polys are non-zero when finalising the circuit, so we shouldn't hit this case.

The reason why this issue arose is that we were using the Merge in the TwoLayerAvmRecursive verifier. That would have failed if we used MergeSettings::Append mode because we would have sent a zero poly for the degree check. Since then we have introduced a specialised form of Goblin for the AVM that doesn't require the Merge.