In Graph make sure is_resource id is string (#3252)

Author: kddejong

src/cfnlint/graph.py

@@ -261,6 +261,8 @@ def _add_edge(self, source_id, target_id, source_path, settings):
 
     def _is_resource(self, cfn, identifier):
         """Check if the identifier is that of a Resource"""
+        if not isinstance(identifier, str):
+            return None
         return cfn.template.get("Resources", {}).get(identifier, {})
 
     def _find_parameter(self, string):