fix: conditional check in non-native IsZero for applying optimization

[ivokub]

Description

In non-native IsZero check we previously checked that every limb is zero, then multiplied the results and returned it. However, as the limbs are significantly smaller than the native field, then we can use the optimization that we add the limbs and perform only a single IsZero check.

However, the optimization we had implemented checks the condition wrong way. This prevents the optimized path to be taken.

This could lead to a underconstrained circuit in extreme cases which are not possible with the emulated parameters defined in gnark (number of limbs has to be greater than the maximal allowed overflow. This means having approximately 190 limbs, but in our parameters the maximum number of limbs is 64).

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules