Handle empty inputs in EvalUnivariate and EvalMultilinearMany

[Galoretka]

Description

• Add early return in EvalUnivariate when len(P)==0 to return 0 (zero polynomial evaluation) instead of panicking on P[0].
• Add early error in EvalMultilinearMany when len(M)==0 to avoid panicking on M[0] and clearly signal invalid input.
• These changes align behavior with existing conventions (e.g., evaluators returning zero for empty input) and harden the API against out-of-range panics, improving robustness without altering normal-path performance.

[Copilot]

Pull Request Overview

This PR adds defensive programming to handle empty inputs in polynomial evaluation methods. The changes prevent panics when empty polynomial arrays are passed and provide appropriate fallback behaviors.

• Adds early return in EvalUnivariate to return zero for empty polynomial coefficients
• Adds early validation in EvalMultilinearMany to return an error for empty multilinear polynomial arrays
• Improves API robustness by preventing index out-of-bounds panics

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[ivokub]

Thanks for the contribution. Looks good.

I think in EvalMultilinearMany we could also return an empty slice and no error. But most probably it would indicate some error upstream when creating inputs, so we could return an error also. Lets see how it works and if other behaviour is needed then can change.