How to produce author_sort using a template but only if it is not "Unknown"?

[geras]

As in title.

At first I thought that the author_sort is set to an empty string and so this template would do the job of naming my books when I save them:

{author_sort:|| - }{title}

but it turns out that it is impossible to set this value to an empty string because then it automatically changed to "Unknown".