chore(evmrpc): return null for unknown/empty block hash

[mojtaba-esk]

Describe your changes and provide context

Fixes PLT-162

Ref: PLT-162

Testing performed to validate your change

[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 13, 2026, 12:04 PM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.58%. Comparing base (c6b1a49) to head (e4f1ad1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3067   +/-   ##
=======================================
  Coverage   58.58%   58.58%           
=======================================
  Files        2080     2080           
  Lines      171656   171660    +4     
=======================================
+ Hits       100567   100573    +6     
+ Misses      62166    62165    -1     
+ Partials     8923     8922    -1     

Flag	Coverage Δ
sei-chain-pr	67.25% <100.00%> (?)
sei-db	70.41% <ø> (ø)

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Files with missing lines	Coverage Δ
evmrpc/block.go	76.38% <100.00%> (+0.33%)	⬆️
evmrpc/utils.go	75.46% <100.00%> (+0.92%)	⬆️

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