Check storage is non-nil before attempting to close it

[masih]

Check that the underlying storage object is non-nil before attempting to close pebble DB. Because, it is possible for the close to be called multiple times, which otherwise will result in panic.

The changes here make the multi-call to Close into noop.

Fixes STO-277

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Jan 6, 2026, 12:07 PM

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.76%. Comparing base (d7d72d9) to head (878021e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sei-db/state_db/ss/test/storage_test_suite.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2659      +/-   ##
==========================================
- Coverage   46.34%   43.76%   -2.58%     
==========================================
  Files        1239     1906     +667     
  Lines      108460   159033   +50573     
==========================================
+ Hits        50262    69603   +19341     
- Misses      53595    83030   +29435     
- Partials     4603     6400    +1797     

Flag	Coverage Δ
sei-chain	45.75% <66.66%> (+<0.01%)	⬆️
sei-cosmos	38.20% <ø> (?)
sei-db	69.13% <100.00%> (+0.06%)	⬆️
sei-tendermint	47.35% <ø> (+0.03%)	⬆️

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

Files with missing lines	Coverage Δ
sei-db/db_engine/pebbledb/mvcc/db.go	64.16% <100.00%> (-0.20%)	⬇️
sei-db/db_engine/rocksdb/mvcc/db.go	58.80% <100.00%> (+0.16%)	⬆️
sei-db/state_db/ss/test/storage_test_suite.go	0.00% <0.00%> (ø)

... and 682 files with indirect coverage changes

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[github-actions]

Successfully created backport PR for release/v6.3:

• Backport release/v6.3: Check storage is non-nil before attempting to close it #2663