store: add missing case for list-keys

[lengau]

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• Have you successfully run pytest tests/unit?

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Cherry-picked hotfix for #4234

CRAFT-1804

[lengau]

Pyright is probably going to fail; that's fixed by #4243

[codecov-commenter]

Codecov Report

Merging #4245 (b50be7d) into hotfix/7.5 (b3dee09) will increase coverage by 0.00%.
The diff coverage is 100.00%.

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@@             Coverage Diff             @@
##           hotfix/7.5    #4245   +/-   ##
===========================================
  Coverage       94.85%   94.85%           
===========================================
  Files             648      649    +1     
  Lines           54355    54379   +24     
===========================================
+ Hits            51556    51580   +24     
  Misses           2799     2799           

Impacted Files	Coverage Δ
tests/legacy/unit/commands/test_list_keys.py	100.00% <ø> (ø)
snapcraft_legacy/_store.py	77.77% <100.00%> (+0.21%)	⬆️
tests/legacy/unit/store/test_keys.py	100.00% <100.00%> (ø)

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