Reset config values after env forcing

[DL6ER]

What does this implement/fix?

Reset config values to their default options if they have been forced through environment and are then unset.

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Related issue or feature (if applicable): pi-hole/docker-pi-hole#1422

Pull request in docs with documentation (if applicable): N/A

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1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
• I signed off all commits. Pi-hole enforces the DCO for all contributions
• I signed all my commits. Pi-hole requires signatures to verify authorship
• I have read the above and my PR is ready for review.

[DL6ER]

Note that the failed spellcheck will be fixed by #1962

[PromoFaux]

Tested