Add default value for $ver in func.php

[yubiuser]

What does this PR aim to accomplish?:

Sets a default value for $ver in func.php used to avoid browser cache issues. Fixes #2496

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[rdwebdesign]

This value can also be an empty string.
What do you think it's better?

Note:
this function will return this default value only if the file doesn't exist, but I think this will happen only if something is really broken, because the web interface should always have all files.

[yubiuser]

Mhh.. you are the web developer. I have no opinion on 0 vs ''. Both are fine for me.

[rdwebdesign]

The question is almost stupid (it's just a preference):

What do you think is better to have in case of "file not found"?

    <link rel="stylesheet" href="style/pi-hole.css?v=0">
    or
    <link rel="stylesheet" href="style/pi-hole.css?v=">
    or something else
    <link rel="stylesheet" href="style/pi-hole.css?v=not_found">

Note:
I still don't know how a user can receive a file not found message from PHP, if AdminLTE ships all used files.

[PromoFaux]

0 is a fine default

[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