Clarify the absence of fields with nested messages

[cthulhu-rider]

There are plenty of fields of message type. I propose to explicitly write in docs whether the field MUST be set.

Where it is not written otherwise, I suggest to require field presence. Later we can relax the requirement, which would be a backwards compatible change.

These requirements will reduce the number of uncertainties when writing software.

Note: this only applies to fields in requests/responses.

[cthulhu-rider]

We decided to consider all nested fields mandatory by default (where would you explicitly write about this?), and if an empty field is allowed, explicitly state this in the containing message.