Fix #7221 (You cannot delete a Series listing from List view)

src/calibre/gui2/library/models.py

@@ -783,6 +783,10 @@ class BooksModel(QAbstractTableModel): # {{{
                     self.db.set_rating(id, val)
                 elif column == 'series':
                     val = val.strip()
+                    if not val:
+                        self.db.set_series(id, val)
+                        self.db.set_series_index(id, 1.0)
+                    else:
                         pat = re.compile(r'\[([.0-9]+)\]')
                         match = pat.search(val)
                         if match is not None: