fix(x/auth): ensure nil .BaseAccounts are reported in ModuleAccount.Validate (backport #1274)

[mergify]

Description

closes: #XXXX

ref. cosmos/cosmos-sdk#16554

Motivation and context

How has this been tested?

Screenshots (if appropriate):

Checklist:

• I followed the contributing guidelines and code of conduct.
• I have added a relevant changelog to CHANGELOG.md
• I have added tests to cover my changes.
• I have updated the documentation accordingly.
• I have updated API documentation client/docs/swagger-ui/swagger.yaml

---

This is an automatic backport of pull request #1274 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of d9f1133 has failed:

On branch mergify/bp/release/v0.48.x/pr-1274
Your branch is up to date with 'origin/release/v0.48.x'.

You are currently cherry-picking commit d9f1133a7.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   x/auth/types/account.go
	modified:   x/auth/types/account_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.78%. Comparing base (cb6669c) to head (0c789a2).

Additional details and impacted files

@@               Coverage Diff                @@
##           release/v0.48.x    #1281   +/-   ##
================================================
  Coverage            69.78%   69.78%           
================================================
  Files                  646      646           
  Lines                67495    67498    +3     
================================================
+ Hits                 47099    47103    +4     
+ Misses               18210    18209    -1     
  Partials              2186     2186           

Files	Coverage Δ
x/auth/types/account.go	76.57% <100.00%> (+0.40%)	⬆️

... and 1 file with indirect coverage changes