Do not return 'false' if IDNA<->UNICODE conversion fails

[yubiuser]

What does this PR aim to accomplish?:

When we try IDNA to unicode conversion (or vice versa) the functions fails if the entered string is not a valid domain (e.g. to long).

https://github.com/pi-hole/AdminLTE/blob/ad2d43869e9ad97cc3a8c5afc6fefec0e142a972/scripts/pi-hole/php/func.php#L629-L666

However, we usually immediately check the returned string right after conversion for validity and return a meaningful error message.

https://github.com/pi-hole/AdminLTE/blob/ad2d43869e9ad97cc3a8c5afc6fefec0e142a972/scripts/pi-hole/php/func.php#L13-L43

The issue occurs, when the conversion fails and no domain is entering validDomain(): it will always fail with the first (negative) matching test ("invalid characters") even if the reason is another one (e.g. too long domain).
(This can be simply tested by trying to add a blacklist entry with one label >63 chars)

This PR mitigates the issue by checking if the conversion failed and returning the non-converted string.

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[PromoFaux]

I like the addition of renaming the functions parameter variable - it makes things easier to read

[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-web-v5-18-2-and-core-v5-15-1-released/60695/1