Support empty commitments

[mimoo]

Currently, commiting to zero polynomials will return an empty commitment:

https://github.com/o1-labs/proof-systems/blob/master/poly-commitment/src/commitment.rs#L594

from @imeckler

The way to really fix it is to add the hiding generator to all the index commitments (like 1*h so the blinding factor is known when we open)

[mimoo]

^ when this is done, we should remove the specific handling of the public commitment https://github.com/o1-labs/proof-systems/pull/717

[github-actions[bot]]

Stale issue message

[mimoo]

I think https://github.com/o1-labs/proof-systems/pull/795 was the last PR that implemented this manually, it would be great to have an API that does this and remove the "manual-ness"