fix: non-native zero IsZero edge case

[ivokub]

Description

When the compiler can deduce that non-native element is constant zero, then we represent it on zero limbs. However in IsZero method we initialized the accumulator from first limb without range checking. This PR adds fast path for constant zero.

Fixes #1412

Type of change

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

How has this been tested?

• Improved IsZeroCircuit test to include the regression

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

[weijiguo]

I have applied the commits and our issues are resolved. Nice job!

[yelhousni]

That makes sense. Fast paths for Div and Sqrt are already implemented in field_ops.go (though the comment here should be when multiplying by 0?).

[ivokub]

That makes sense. Fast paths for Div and Sqrt are already implemented in field_ops.go (though the comment here should be when multiplying by 0?).

Thanks - changed the comment from "divide by 0" to "divide 0".