[Server] Protobuf spec should use Field Presence

[Kok Rui Wong]

As of commit c6b9ac5 the MUI Server now uses protobuf version ~=3.20

This means that we can safely support Explicit Field Presence (https://github.com/protocolbuffers/protobuf/blob/main/docs/field_presence.md) - most notably by denoting specific fields as optional - this helps make the proto spec clearer for devs, and exposes the HasField API method so we don't have to manually set defaults to make them unambiguous