[Proof] Improve proptests
[Proof] Improve proptests

Improve `Arbitrary` implementation for `SparseMerkleProof`, similar to
https://github.com/libra/libra/pull/1283.

Also there was a bug in https://github.com/libra/libra/pull/1283 where
the strategy no longer generates proofs of size 0. Fix that as well.

Closes: #1300
Approved by: msmouse

aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra