fix: nil check in post (backport #118)

[mergify]

Closes #114
Closes #115

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This is an automatic backport of pull request #118 done by Mergify.

[mergify]

Cherry-pick of f0997fb has failed:

On branch mergify/bp/sdk-47/pr-118
Your branch is up to date with 'origin/sdk-47'.

You are currently cherry-picking commit f0997fb.
  (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:   docs/INTEGRATIONS.md
	modified:   x/feemarket/ante/fee.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   x/feemarket/post/fee.go
	both modified:   x/feemarket/post/fee_test.go

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