Fix ValidatorSetFromProto crash on nil proposer in stored state

[masih]

Describe your changes and provide context

Fixes the node startup crash:

Error: error creating node: error="fromProto: validatorSet proposer error: nil validator" closerError=""

Root cause

There is a serialization/deserialization asymmetry between ValidatorSet.ToProto() and ValidatorSetFromProto():

• ToProto() returned &tmproto.ValidatorSet{} (non-nil, empty proto with nil Proposer) when the validator set was nil or empty
• ValidatorSetFromProto() checks if the proto is nil, but did not handle a non-nil proto with a nil Proposer field — it unconditionally called ValidatorFromProto(vp.GetProposer()), which fails when GetProposer() returns nil

This means any state saved with a nil/empty ValidatorSet (e.g. LastValidators during bootstrap, state sync edge cases, or proto version mismatches across binary upgrades) produced a proto that could never be loaded back, preventing the node from starting.

Fix (two sides)

1. Root cause — ValidatorSet.ToProto(): Return nil instead of &tmproto.ValidatorSet{} for nil/empty sets. A nil proto field is correctly handled everywhere: proto serialization omits it, and ValidatorSetFromProto(nil) returns a clear "nil validator set" error. The old empty-but-non-nil proto silently succeeded on write but crashed on read.

2. Backward compat — ValidatorSetFromProto(): Handle nil Proposer gracefully for already-persisted data. If the proto has validators but no Proposer, derive the proposer using findProposer() (same fallback already used by GetProposer()). If both are absent, ValidateBasic() returns ErrValidatorSetEmpty.

3. Propagation — LightBlock.ToProto(): Return ErrValidatorSetEmpty when a present-but-empty ValidatorSet serializes to nil, catching semantically invalid light blocks at serialization time.

Testing performed to validate your change

• All existing tests pass (types, state, light, statesync packages)
• Added TestValidatorSetFromProto_NilProposerWithValidators: constructs a proto with valid validators but nil Proposer, verifies the proposer is correctly derived and is a member of the set
• Added TestValidatorSetFromProto_EmptyProtoRoundtrip: verifies ToProto on empty set returns nil and FromProto(nil) returns error
• Updated TestLightBlockProtobuf to expect ToProto error on empty validator sets (previously only caught at FromProto time)
• gofmt -s -l produces no output

  

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 21, 2026, 7:04 PM

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.59%. Comparing base (6d0e8dc) to head (ede6d84).

Files with missing lines	Patch %	Lines
sei-tendermint/types/validator_set.go	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3102   +/-   ##
=======================================
  Coverage   58.58%   58.59%           
=======================================
  Files        2096     2096           
  Lines      173413   173418    +5     
=======================================
+ Hits       101601   101616   +15     
+ Misses      62757    62748    -9     
+ Partials     9055     9054    -1     

Flag	Coverage Δ
sei-chain-pr	81.97% <80.00%> (?)
sei-db	70.41% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sei-tendermint/types/light.go	82.52% <100.00%> (+2.32%)	⬆️
sei-tendermint/types/validator_set.go	94.06% <75.00%> (+0.96%)	⬆️

... and 2 files with indirect coverage changes

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