Welcome to the forum dudas.
I'll admit I didn't look at the post in great detail (and why the giant and alternating font sizes?) but I am wondering why you use a dot?, and I notice you are using some unnecessary brackets.
instead of
$IsNull(<Artists: Guest>,.," (feat. "<Artists: Guest>")")
this would probably work better
$IsNull(<Artists: Guest>,,<Artists: Guest>)
But this has left out the 'feat' part, and perhaps it doesn't achieve what you want exactly?
But hopefully this was some useful information for you to proceed.