return error when validating an empty batch

[conradoplg]

Closes #445
Closes #446

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03% 🎉

Comparison is base (87346f4) 77.46% compared to head (e98bc8d) 77.49%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #487      +/-   ##
==========================================
+ Coverage   77.46%   77.49%   +0.03%     
==========================================
  Files          30       30              
  Lines        2738     2742       +4     
==========================================
+ Hits         2121     2125       +4     
  Misses        617      617              

Files Changed	Coverage Δ
frost-core/src/batch.rs	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[dconnolly]

💯