remove 'unknown' in url when tag is empty

Hello guys,
I have a problem regarding permalink structure. Currently my structure is

/%category%/%band%/%postname%

as category is ‘lyric’, ‘music video’ etc
‘band’ is the corresponding band name

Now I have another category which doesn’t concern to bands at all so when my custom post type ‘band’ is empty there is an ‘unknown’ in the url which I want to remove. Is there any way to put a if else operator in the permalink structure or any other way of removing empty values?