Align LightBlockFromProto with its doc comment

[masih]

The doc comment for LightBlockFromProto promises an error when either the validator set or the signed header is invalid. The implementation, however, treated nil sub-fields as optional and returned a partially populated LightBlock. Honor the documented contract so callers can rely on err == nil meaning the returned LightBlock is fully populated.

Also fix the "Lightblock" -> "LightBlock" typo in the same comment.

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	May 19, 2026, 8:35 AM

[codecov]

Codecov Report

❌ Patch coverage is 76.31579% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.31%. Comparing base (5ac709d) to head (04a8155).

Files with missing lines	Patch %	Lines
sei-tendermint/types/light.go	76.31%	8 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3460   +/-   ##
=======================================
  Coverage   59.30%   59.31%           
=======================================
  Files        2127     2127           
  Lines      175876   175832   -44     
=======================================
- Hits       104305   104294   -11     
+ Misses      62473    62458   -15     
+ Partials     9098     9080   -18     

Flag	Coverage Δ
sei-chain-pr	81.56% <76.31%> (?)
sei-db	70.41% <ø> (ø)

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Files with missing lines	Coverage Δ
sei-tendermint/types/light.go	78.43% <76.31%> (-1.77%)	⬇️

... and 31 files with indirect coverage changes

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