Do not include empty values in generated JSON.
Enable DocletOptions to be constructed without RootDoc.

This is to enable easier testing; and there is no reasonable requirement
on binding it to RootDoc since it has reasonably OK defaults.
Fix type resolving bug with Types not having ClassDocs.

It also fixes the default return type for primitives; these classes
should not know how the fallback naming should operate and should just
return null if there are any problems.
Return rootPath from ApiClassParser to be used by ApiDeclaration.

This saves up the effort of trying to manually calculate it for the
ApiDeclaration instance. The API declaration is roughly the equivalent
of the Resource class, so the resource's @Path can be used as root.
Tests: adding tests for primitive types.