Fix a few PHP 8 warnings

[rdwebdesign]

What does this PR aim to accomplish?

Fix #2562

How does this PR accomplish the above?

Making sure the variables/values are set before using them.

Link documentation PRs if any are needed to support this PR:

none

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[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/the-settings-page-is-not-displayed/62168/8