fix(marshmallow): address bad import (#51)

closes #50

Author: Matt Topol

quart_openapi/marshmallow.py

@@ -21,3 +21,7 @@ def schema_to_json(schema: Schema):
     JSONSchema = None
     MarshmallowValidationError = None
     MARSHMALLOW = False
+
+    def schema_to_json():
+        """provide empty implmentation for the case where
+        marshmallow importing failed."""