Annotate with @JsonClass classes with no parameters/variables

[macisamuele]

This is needed because if we would fully remove moshi-kotlin dependency (as planned by version 2 release) we would not be able to properly have the Json to Object conversion and vice-versa.

The most important change introduced by this PR is visible into samples/junit-tests/.../models/EmptyModel.kt.

The reason for forcing the presence of @JsonClass(generateAdapter = true) even if there are no attributes is related to the fact that if we would remove the usage of KotlinJsonAdapterFactory then we would not be able to covert the EmptyModel instance from/to JSON.

In order to validate it I've run this script.
The script basically ensures that we have no moshi-kotlin/kotlin-reflect dependencies on samples/junit-tests and runs the tests

The results (raw results in here), as expected, shows that:

• PR branch leads to green tests
• current master fails to run emptyEndpointTest_withEmptyBody test.
  The test failure contains the following exception (the raw exception is available here)

java.lang.IllegalArgumentException: Unable to create converter for class com.yelp.codegen.generatecodesamples.models.EmptyModel
...
Caused by: java.lang.IllegalArgumentException: Cannot serialize Kotlin type com.yelp.codegen.generatecodesamples.models.EmptyModel. Reflective serialization of Kotlin classes without using kotlin-reflect has undefined and unexpected behavior. Please use KotlinJsonAdapter from the moshi-kotlin artifact or use code gen from the moshi-kotlin-codegen artifact.

[cortinico]

Looks good but please update the tests. Also having #61 fixed would be great to catch this scenario in the future.

[cortinico]

LGTM!