prevent parsing empty uri

Author: pvgenuchten

csvwlib/converter/ToRDFConverter.py

@@ -82,7 +82,7 @@ def parse_virtual_columns(self, row_node, atdm_row, table_metadata):
                     self.graph.add((row_node, CSVW.describes, subject))
             else:
                 print(f"term {virtual_column['propertyUrl']} not in namespaces")
-S
+
     def _add_file_metadata(self, metadata, node):
         language = JSONLDUtils.language(self.metadata.get('@context',[]))
         for key, value in metadata.items():
@@ -165,6 +165,8 @@ def _object_node(value, col_metadata, atdm_row, tm_url):
 
     @staticmethod
     def _normalize_to_uri(string):
+        if string in [None, '']:
+            return ''
         string = quote(string, safe='')
         string = string.replace('-', '%2D')
         return string