fix: align GetCoinbaseAddress to handle empty proposer address

[mmsqe]

in contexts like CheckTx

Description

Closes: #XXXX

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Author Checklist

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I have...

• tackled an existing issue or discussed with a team member
• left instructions on how to review the changes
• targeted the main branch

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.20%. Comparing base (3225549) to head (9316d15).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #817      +/-   ##
==========================================
+ Coverage   64.96%   65.20%   +0.23%     
==========================================
  Files         317      317              
  Lines       21604    21604              
==========================================
+ Hits        14036    14086      +50     
- Misses       6349     6357       +8     
+ Partials     1219     1161      -58     

Files with missing lines	Coverage Δ
x/vm/keeper/block_proposer.go	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

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