fix: don't fail post handler on simulate tx with no fee (backport #122)

[mergify]

• add similar logic as bug: Don't fail ante handler on simulate tx with no fee #120 in post handler
• additional unit testing
• gas estimation remains the same as it was before this PR from simulating test Tx

  ---

  This is an automatic backport of pull request fix: don't fail post handler on simulate tx with no fee #122 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   README.md
	modified:   x/feemarket/ante/fee.go
	modified:   x/feemarket/ante/fee_test.go
	modified:   x/feemarket/post/fee_test.go

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