fix: add missing nil check in store.GetStore

[robert-zaremba]

Description

Both store.GetStore and store.GetKVStore are missing nil checks which yield to more ugly panics.

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• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #9354 (79fa51f) into master (9d05048) will decrease coverage by 0.07%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #9354      +/-   ##
==========================================
- Coverage   60.38%   60.31%   -0.08%     
==========================================
  Files         587      588       +1     
  Lines       36920    36976      +56     
==========================================
+ Hits        22294    22301       +7     
- Misses      12651    12699      +48     
- Partials     1975     1976       +1     

Impacted Files	Coverage Δ
store/rootmulti/store.go	68.23% <50.00%> (-0.24%)	⬇️
store/cachemulti/store.go	11.32% <80.00%> (ø)