Change directory parameter to optional

[steveny91]

What does this PR do?

This PR makes the directory parameter in the postfix check optional to keep backwards compatibility.

Motivation

In check version < 1.9, there wasn't any validation for the check, which allowed the check to work without setting a directory parameter. In the newest version (1.9) there's a validator that checks that directory is set and this breaks existing configuration in which postqueue: true is set and directory is omitted.

Additional Notes

Also added some explaining text in the conf.yaml.example and updated the readme accordingly.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[github-actions]

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[codecov]

Codecov Report

Merging #10051 (cb9ad3f) into master (b7f9e13) will increase coverage by 0.04%.
The diff coverage is n/a.

Flag	Coverage Δ
postfix	88.04% <ø> (ø)

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[github-actions]

The validations job has failed; please review the Files changed tab for possible suggestions to resolve.

[alai97]

Looks good pending the following suggestions (removing "please")!