Treat author_sort = _('Unknown') as null

src/calibre/ebooks/metadata/book/base.py

@@ -32,6 +32,7 @@ def human_readable(size, precision=2):
                 'device_collections': [],
                 'author_sort_map': {},
                 'authors'      : [_('Unknown')],
+                'author_sort'  : _('Unknown'),
                 'title'        : _('Unknown'),
                 'user_categories' : {},
                 'author_link_map' : {},

src/calibre/library/save_to_disk.py

@@ -173,8 +173,6 @@ def get_components(template, mi, id, timefmt='%b %Y', length=250,
     tsorder = tweaks['save_template_title_series_sorting']
     format_args = FORMAT_ARGS.copy()
     format_args.update(mi.all_non_none_fields())
-    if mi.author_sort == _('Unknown'):
-        format_args['author_sort'] = _('Unknown')
     if mi.title:
         if tsorder == 'strictly_alphabetic':
             v = mi.title