Fix empty prevote latency metrics

[codchen]

Describe your changes and provide context

The node would crash if there is no prevotes info for a previous round

https://linear.app/seilabs/issue/SEI-5251/tendermint-rpc-bug

Testing performed to validate your change

tested with block sync without round info

[codecov]

Codecov Report

Merging #154 (446fd45) into main (d6af34b) will decrease coverage by 0.30%.
Report is 2 commits behind head on main.
The diff coverage is 57.14%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #154      +/-   ##
==========================================
- Coverage   59.16%   58.86%   -0.30%     
==========================================
  Files         281      281              
  Lines       38797    38799       +2     
==========================================
- Hits        22954    22840     -114     
- Misses      14061    14164     +103     
- Partials     1782     1795      +13     

Files Changed	Coverage Δ
internal/consensus/state.go	70.02% <57.14%> (-2.15%)	⬇️

... and 16 files with indirect coverage changes