Clarify metadata deletion message.

[dcr-stripe]

r? @richardm-stripe

Summary

Clarifies the message when we try to mutate a metadata value to '' directly on a StripeObject.

• When using save(), = None should be used instead to delete.
• When using modify(), users should pass in '' to delete.

Motivation

#753

This will hopefully clear up some clarification for the above issue when mixing paradigms.