Adds Failing Test Case to Test

tests/test_schema_converter.py

@@ -204,6 +204,9 @@ class TestSchemaConverter(TestCase):
             model(friends=["John", "Jane", "John"]).friends, {"John", "Jane"}
         )
 
+        with self.assertRaises(ValueError):
+            model()
+
         with self.assertRaises(ValueError):
             model(friends=[])