fix: guard nil light client attack signed header

[UnbornAztecKing]

Summary

• Add a nil SignedHeader guard before LightClientAttackEvidence accesses the promoted Header field.
• Add direct validation and protobuf decode regressions for omitted conflicting_block.signed_header.

Validation

• go test ./types -run TestLightClientAttackEvidenceValidation
• go test ./types -run 'TestLightClientAttackEvidence|TestEvidenceFromProto'
• go test ./types
• go test ./consensus ./state ./types