fix: app: defensively check for nil tx

[arajasek]

Just adding a defensive check in case something funky happens here.

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Dec 17, 2025, 6:24 PM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.61%. Comparing base (217ae36) to head (f245578).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2628   +/-   ##
=======================================
  Coverage   43.61%   43.61%           
=======================================
  Files        1899     1899           
  Lines      158389   158391    +2     
=======================================
+ Hits        69078    69085    +7     
  Misses      82962    82962           
+ Partials     6349     6344    -5     

Flag	Coverage Δ
sei-chain	45.51% <100.00%> (-0.03%)	⬇️
sei-cosmos	38.03% <ø> (+<0.01%)	⬆️
sei-db	69.06% <ø> (ø)
sei-tendermint	47.51% <ø> (+0.05%)	⬆️

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Files with missing lines	Coverage Δ
app/app.go	70.88% <100.00%> (-1.05%)	⬇️

... and 20 files with indirect coverage changes

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[github-actions]

Successfully created backport PR for release/v6.3:

• Backport release/v6.3: fix: app: defensively check for nil tx #2629