Make explicit the validity of the empty set in MerkleProof.sol

[ernestognw]

Makes it explicit that users verifying multiproofs should consider that the empty set is valid.
Most implementations do check the validity of the leaves or build them within the contract, which makes it impossible to validate a proof incorrectly.

This is consistent with our no-ops policy

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[cairoeth]

lgtm, would be cherry-picked for 5.1 update?

[ernestognw]

lgtm, would be cherry-picked for 5.1 update?

Yes, I'll pick it up to #5139