When bulk editing custom series-type columns and not provding a series number use 1 as the default, instead of None

src/calibre/library/custom_columns.py

@@ -532,6 +532,8 @@ def _set_custom(self, id_, val, label=None, num=None, append=False,
 
         if data['datatype'] == 'series' and extra is None:
             (val, extra) = self._get_series_values(val)
+            if extra is None:
+                extra = 1.0
 
         books_to_refresh = set([])
         if data['normalized']: