feat!: handles empty proofs in validateLeafHash and VerifyInclusion

[staheri14]

Overview

Closes #140
The changes made in this PR are considered to be breaking, as they modify the behavior of VerifyInclusion. In the prior version, the VerifyInclusion function would return false for the first test case named "valid empty proof and leaves == empty." of the TestVerifyInclusion_EmptyProofs suite. However, with the changes implemented, an empty proof for an empty set of leaves is now considered a valid proof. As a result, VerifyInclusion now returns true.
cc: @liamsi

Checklist

• New and updated code has appropriate documentation
• New and updated code has new and/or updated testing
• Required CI checks are passing
• Visual proof for any user facing features like CLI or documentation updates
• Linked issues closed with keywords

[codecov]

Codecov Report

Merging #184 (fc595aa) into master (f05092f) will increase coverage by 1.51%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
+ Coverage   94.14%   95.66%   +1.51%     
==========================================
  Files           5        5              
  Lines         547      554       +7     
==========================================
+ Hits          515      530      +15     
+ Misses         18       14       -4     
+ Partials       14       10       -4     

Impacted Files	Coverage Δ
proof.go	93.70% <100.00%> (+6.20%)	⬆️

[evan-forbes]

LGTM