/profile body parameters: required not omittable

This is a simple clarification hopefully not needing too much time. The current API definition says that `displayname` and `avatar_url` in `/profile/{}/displayname` and `/profile/{}/avatar_url` respectively are not required. However, Synapse complains - and I think rightfully - when I completely omit the parameter (in a wrong way but that's a [separate issue](https://github.com/matrix-org/synapse/issues/8029)). Should Synapse or the spec be fixed?

Compare the API definition for `/profile/{}/displayname` and `/profile/{}/avatar_url` with Synapse's handling of omitted body parameters. Determine whether the specification or Synapse should define these parameters as required, then update the relevant clarification so the endpoint behavior and ownership are consistent.

Written by the indexing model from the issue text.