Fix #1380

Add No Prefix and Any Prefix filter/sort options.
https://github.com/mybb/mybb/issues/1380
> False. hint*guests*hint

Now you're mistaking all users with 'guests'... Why don't you understand a simple thing that the isset() can't just be used because it results in a bug? Add isset() back and you'll see that it will be "From Today" for [b]registered users[/b] by default, which is not intended.

!empty() is a completely different thing - it checks isset() [b]and[/b] the value itself. I'll change it to !empty() if you want to fix the possible PHP notice, but I won't revert to a buggy code...

> Why don't you stop making assumptions you are not qualified to do?

I'm not qualified to do because of what? I can surely leave the team if I'm not qualified to be in it and post my opinion.

> Or at least be, don't know, kind?

Sorry if I sounded too harsh, but the same applies to you.