The `constructFromObject` factory methods should be class methods
(or "static" methods), not instance methods.
With this commit, ApiClient no longer calls the model constructors
directly. Instead, it calls the new static factory method to get the
new instance. If there is no data on the top level, null is returned.
It is still possible for users to call the model constructors
directly, of course.
Fixes#2103.
In the constructFromObject methods, we were always setting all the
parameters, including optional ones that were missing in the `data`
object. Because of the `convertToType` calls, this had the effect
of producing the *strings* `"undefined"` or `"null"` as property
values on the constructed object! This obviously leads to difficult
bugs.
This commit makes sure that we first check that the data field
exists in the data object. Note that both `null` and `undefined`
values will be skipped.
