Prevent panicking each time the computed sum is zero

[pierreeliseeflory]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Docs change / refactoring / dependency upgrade

Motivation and Context / Related issue

In Private Join and Compute, a sum of features equal to zero (like for instance if the intersection is empty) made the code panicking because of BigUint::to_u64_digits returning an empty vec when converting a BigUint::Zero.

Checklist

• The documentation is up-to-date with the changes I made.
• I have read the CONTRIBUTING document and completed the CLA (see CONTRIBUTING).
• All tests passed, and additional code has been covered with new tests.