merkle: return rfc6962 hash for empty merkle tree

[liamsi]

fixes #498
related golang changes: tendermint/tendermint#5193

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG.md

[codecov-commenter]

Codecov Report

Merging #514 into master will increase coverage by 0.1%.
The diff coverage is 56.2%.

@@           Coverage Diff            @@
##           master    #514     +/-   ##
========================================
+ Coverage    29.5%   29.7%   +0.1%     
========================================
  Files         135     135             
  Lines        5468    5481     +13     
  Branches     1700    1698      -2     
========================================
+ Hits         1617    1630     +13     
  Misses       2792    2792             
  Partials     1059    1059             

Impacted Files	Coverage Δ
tendermint/src/merkle.rs	57.6% <56.2%> (+2.0%)	⬆️
light-client/src/tests.rs	49.5% <0.0%> (+0.8%)	⬆️
light-client/src/peer_list.rs	50.9% <0.0%> (+1.2%)	⬆️
light-client/src/state.rs	45.0% <0.0%> (+5.0%)	⬆️

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[xla]

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