Use an empty string instead of None when the text of the set-value el…

…ement is empty

openscap_report/scap_results_parser/scap_results_parser.py

@@ -52,7 +52,7 @@ def validate(self, xsd_path):
 
     def _get_ref_values(self):
         return {
-            ref_value.get("idref"): ref_value.text
+            ref_value.get("idref"): ref_value.text if ref_value.text is not None else ""
             for ref_value in self.root.findall('.//xccdf:set-value', NAMESPACES)
         }