refactor(x/auth): allow empty public keys for GetSignBytesAdapter (backport #19651)

[mergify]

Description

to avoid error like in #19094

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Author Checklist

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please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• reviewed "Files changed" and left comments if necessary
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• updated the relevant documentation or specification, including comments for documenting Go code
• confirmed all CI checks have passed

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• confirmed the correct type prefix in the PR title
• confirmed all author checklist items have been addressed
• reviewed state machine logic, API design and naming, documentation is accurate, tests and test coverage

Summary by CodeRabbit

• New Features
  • Enhanced the authentication mechanism to support transactions without public keys, improving flexibility and use cases.

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This is an automatic backport of pull request #19651 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 06a3989 has failed:

On branch mergify/bp/release/v0.53.x/pr-19651
Your branch is up to date with 'origin/release/v0.53.x'.

You are currently cherry-picking commit 06a398931.
  (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)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md
	both added:      x/auth/signing/adapter_test.go

no changes added to commit (use "git add" and/or "git commit -a")

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

[julienrbrt]

Changes are non functional, so it was already there. Closing.

[github-actions]

⚠️ govulncheck found vulnerabilities:

mkdir -p /home/runner/work/cosmos-sdk/cosmos-sdk/build/
GOBIN=/home/runner/work/cosmos-sdk/cosmos-sdk/build go install golang.org/x/vuln/cmd/govulncheck@latest
go: downloading golang.org/x/vuln v1.1.4
go: downloading golang.org/x/telemetry v0.0.0-20240522233618-39ace7a40ae7
go: downloading golang.org/x/mod v0.22.0
go: downloading golang.org/x/tools v0.29.0
/home/runner/work/cosmos-sdk/cosmos-sdk/build/govulncheck ./...
govulncheck: loading packages: 
There are errors with the provided package patterns:

x/auth/signing/adapter_test.go:9:1: missing import path

For details on package patterns, see https://pkg.go.dev/cmd/go#hdr-Package_lists_and_patterns.

make: *** [Makefile:155: vulncheck] Error 1