Inconsistency in which profile fields are returned and what values they get

Querying for the display name of someone who does not have a display name gets you `{displayname: null}`, however if you query for a field that does not exist you get `{}`. Likewise, the full profile for a user that has nothing set is `{displayname: null, avatar_url: null}`.

Should decide on whether `null` is used, or just omitting the properties.

Review the Matrix profile-field response behavior described in the issue and determine whether absent values should be represented as null or omitted. No file or test is named; the work is done when the project has resolved the policy and the relevant Matrix specification text consistently documents that behavior.

Written by the indexing model from the issue text.