Clarify handling of optional fields.

[sirthias]

The Hetzner Cloud API definition defines quite a few request parameters that are optional.
Currently it is unclear how one can define such a parameter as "not set".
Should it be set to null? Are there parameters where the meaning of null differs from not set?

Ideally the fact whether a parameter is optional or not would be expressed through the type system, e.g. via something like Java 8's Optional type.