fix empty records when some columns are empty

plugin.py

@@ -376,14 +376,14 @@ class DatastorePlugin(p.SingletonPlugin):
         records_format = data_dict.get(u'records_format')
         json_values = records_format in (u'objects', u'lists')
         for field_id in field_ids:
-            fmt = u'to_json({0})' if records_format == u'lists' else u'{0}'
+            fmt = u"COALESCE(to_json({0}), json 'null')" if records_format == u'lists' else u'{0}'
             typ = fields_types.get(field_id)
             if typ == u'nested':
                 fmt = u'({0}).json'
             elif typ == u'timestamp':
                 fmt = u"to_char({0}, 'YYYY-MM-DD\"T\"HH24:MI:SS')"
                 if json_values:
-                    fmt = u"to_json({0})".format(fmt)
+                    fmt = "COALESCE(to_json({0}), json 'null')".format(fmt)
             elif typ.startswith(u'_') or typ.endswith(u'[]'):
                 fmt = u'array_to_json({0})'