handle config with xpath null/None

Author: ashwinrajeev

selectorlib/selectorlib.py

@@ -73,7 +73,7 @@ def extract(self, html: str, base_url: str = None):
         return fields_data
 
     def _extract_selector(self, field_config, parent_parser):
-        if 'xpath' in field_config:
+        if field_config.get("xpath") is None:
             elements = parent_parser.xpath(field_config['xpath'])
         else:
             css = field_config['css']