fix: allow empty public keys when setting signatures (backport #19106)

[mergify]

This is an automatic backport of pull request #19106 done by Mergify.
Cherry-pick of e621eb6 has failed:

On branch mergify/bp/release/v0.50.x/pr-19106
Your branch is up to date with 'origin/release/v0.50.x'.

You are currently cherry-picking commit e621eb6b1.
  (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/tx/builder.go
	modified:   x/auth/tx/builder_test.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   x/auth/CHANGELOG.md

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

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[julienrbrt]

note for myself, removing this from main at v0.50.4 tag or v0.47.8 tag