Use get to safely pull readOnlyProperties. (#3141)

Co-authored-by: Kevin DeJong <[email protected]>

src/cfnlint/template/template.py

@@ -423,7 +423,7 @@ def build_output_string(resource_type, property_name):
                         for schema in cfnlint.helpers.REGISTRY_SCHEMAS:
                             if value["Type"] == schema["typeName"]:
                                 results[name] = {}
-                                for ro_property in schema["readOnlyProperties"]:
+                                for ro_property in schema.get("readOnlyProperties", []):
                                     try:
                                         item = resolve_pointer(schema, ro_property)
                                     except KeyError: