Always use !== -1 to check for a string's existence

[XhmikosR]

String.indexOf() returns -1 if the string isn't found so this is consistent with the rest of the codebase.

Signed-off-by: XhmikosR [email protected]

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

https://discourse.pi-hole.net/t/pi-hole-5-1-released/35577/1