Report and exit if dig supportedOS returns nothing

[yubiuser]

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• I have made only one major change in my proposed changes.
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What does this PR aim to accomplish?:
If dig for supportedOS does not return any value (e.g. due to misconfigured upstream DNS servers) it should report an appropriate error and exit the script

How does this PR accomplish the above?:
Check if supportedOS is an empty array.

What documentation changes (if any) are needed to support this PR?:
No changes required

[PromoFaux]

Rebase please, following #3596

[yubiuser]

Done. (I think...)

[pralor-bot]

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

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-2-released/36964/1