Which fields are returned in a response?

Many methods in the Stack Exchange API return complicated responses.
For example, /questions?answers=true&comments=true&... returns some set of questions (which may have comments) with answers (which may, themselves, have comments) and any returned question, answer, or comment will likely contain some information on a user. Parsing this data can be complicated by uncertainty about which fields will be returned.

To clarify, unless contra-indicated by a parameter (as body, comments, and answers often are), the API will make a best effort attempt at providing a field. If data is not available, the field will be omitted from the return. The API will never return null, the field will simply not be present.

This affects "shallow user" fields most frequently.
For example, this question's owner has been deleted (a very rare occurrence). An API query for that question returns no user accordingly.