Wrt field nullability - we should try using the proto optional keyword and proto3 field presence rules to inform generation. So:

• optional = nullable
• not optional = not nullable
• not optional means ints, string, enums get default values
• required (url path params, body contents) mean non-nullable for message fields
• potentially, lists and maps are never null (but get defaults if necessary - [] and {})?

Originally posted by @devoncarew in #1034