fields: don't crash storing items field if there's no options (#40536)

wcs/fields.py

@@ -1704,8 +1704,9 @@ class ItemsField(WidgetField):
         return values
 
     def store_display_value(self, data, field_id):
-        kwargs = {}
         options = self.get_options()
+        if not options:
+            return ''
         choices = []
         for choice in data.get(field_id) or []:
             if type(options[0]) is str: