Use nil instead of protobuf.Empty in genesis BaseAccount

[amaury1093]

Description

Accounts in the 0.37 exported state don't have pubkeys. When migrating these accounts to 0.40, we were packing a protobuf's Empty inside the BaseAccount's pubkey Any field.

It would be much simpler to just pack a nil, which outputs as null in the JSON.

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• 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 #7463 into master will increase coverage by 2.43%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7463      +/-   ##
==========================================
+ Coverage   55.25%   57.68%   +2.43%     
==========================================
  Files         591      288     -303     
  Lines       36948    19575   -17373     
==========================================
- Hits        20414    11292    -9122     
+ Misses      14426     7217    -7209     
+ Partials     2108     1066    -1042