Guard socket and FTL response

[yubiuser]

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes.
• I am willing to help maintain this change if there are issues with it later.
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• It is compatible with the EUPL 1.2 license
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What does this PR aim to accomplish?:
When the FTL socket dies unexpectedly (i.e. by Pi-hole restart) and the web interface is still open, $data = array("FTLnotrunning" => true); is send to the calling functions. However, there was no guarding so far for this response, which leads to errors within the calling functions. This PR adds a check for the $data = array("FTLnotrunning" => true); response and only proceeds the calling function if this data is not present.
Additionally, it guards the fclose($socket); by checking for is_resource($socket))

[rdwebdesign]

I understand the reason for this change, but I have 2 questions:

What is returned to javascript, if PHP founds "FTLnotrunning"? Nothing?
How does javascript handle this kind of error?

[yubiuser]

Good question. So far I just return nothing. But I looked and most calls in js are guarded with

    if ("FTLnotrunning" in data) {
      return;
    }

So I think a nice solution is to return an array with FTLnotrunning => true and make sure we have such a guard as above.

[rdwebdesign]

I agree.
JS is already expecting a "complete return value" or FTLnotrunning => true.

[yubiuser]

Added the remaining js guard for php returning FTLnotrunning to queries.js. It will notify the user and send an empty array data to datatables resulting in an empty query log.

[yubiuser]

The tests will pass after #2166 has been merged and the branch has been rebased.

[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-v5-15-web-v5-12-and-core-v5-10-released/54987/1