feat!: rejects invalid empty range proofs

[staheri14]

Overview

Resolves https://github.com/celestiaorg/nmt/issues/164

Checklist

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

[codecov[bot]]

Codecov Report

Merging #180 (9e3d8de) into master (0c70c1b) will increase coverage by 0.05%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   94.04%   94.09%   +0.05%     
==========================================
  Files           5        5              
  Lines         537      542       +5     
==========================================
+ Hits          505      510       +5     
  Misses         18       18              
  Partials       14       14              

Impacted Files	Coverage Δ
proof.go	87.50% <100.00%> (+0.47%)	:arrow_up:

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