title: Pydantic: Detect if a field value is missing or given as null
body: <p>I want to allow users to selectively update fields using PUT calls. On the pydantic model, I have made the fields Optional. In the FastAPI handler if the model attribute is None, then the field was not given and I do not update it.</p>
  <p>The problem with this approach is that there is no way for the client to &quot;blank out&quot; a field that isn't required for certain types.</p>
  <p>In particular, I have date fields that I want clients to be able to clear by sending in a null in the JSON. How can I detect the difference between the client sending null or the client not sending the field/value at all? The model attribute is just None in either case.</p>