sanityCheck: only necessarily calculate siblingHash if len(proof.SideNodes > 0 but also return early and unnest

[odeke-em]

If we look at the code in here https://github.com/celestiaorg/smt/blob/a99c0f5249884312ab8e6864fe165371c0f461ba/proofs.go#L44-L54

Notice

• We are calculating a cryptographic hash even if len(proof.SideNodes) conditions could cause a return https://github.com/celestiaorg/smt/blob/a99c0f5249884312ab8e6864fe165371c0f461ba/proofs.go#L46-L48

• Those nested conditions could be further simplified from

  Before

  https://github.com/celestiaorg/smt/blob/a99c0f5249884312ab8e6864fe165371c0f461ba/proofs.go#L45-L52

  After

      // Check that the sibling data hashes to the first side node if not nil
      if proof.SiblingData == nil {
              return true
      }
  
      if len(proof.SideNodes) == 0 {
              return true
      }
  
      siblingHash := th.digest(proof.SiblingData)
      return bytes.Equal(proof.SideNodes[0], siblingHash)