[java] Referenced schema is not generated properly

Description

After applying the fix for issue #1624 (PR #1625), the attached schema results in incorrect code generation for the "java" generator, although I suspect other generators would suffer from the same fate.
Prior to applying the fix for #1624, the generator throws an NPE :)

The crux of the problem is that the java generator will generate the "Message" class as expected, with the "context" field being declared as a "Context" object. Unfortunately, the Context model is not generated because it is apparently considered to be a top-level map.

One might think that a possible solution would be to generate the Message.context field so that it is of type Map<String, Object> since the Context model is optimized away after being interpreted as a top-level map. But in my opinion, since Context is a referenced schema, we should not be optimizing that away. It is a distinct model defined in the apispec as an object that can have arbitrary properties set on it and is referenced from a schema property belonging to the Message schema. Therefore, it should be rendered as such... i.e. we should generate a class called Context which is a subclass of Map<String, Object>. At the very least, there should probably be a configuration property that can enable/disable the feature introduced by issue #1296.

I could see this schema flattening being done if the Message.context schema property's schema was an inline schema, but not in the case where Message.context's schema is a reference to another named schema.

openapi-generator version

master (latest)

OpenAPI declaration file content or url

Here's the apispec which exhibits the problem:
testcase.zip

Command line used for generation

Using latest master code + fix for issue #1624:

java -jar <location-of-jar>/openapi-generator-cli.jar generate -i testcase.json -g java -o ./java/testcase

Steps to reproduce

1. Process the apispec using the command above
2. Observe the emitted Message.java class; the "context" field is of type Context, but the Context model did not get generated.
   Note: if the fix for #1624 is not applied, then the result will be an NPE

Related issues/PRs

#1296

Suggest a fix/enhancement

Either revert the changes in #1296 or support a configuration property that can disable that feature.

Run the Java generator command against the attached testcase.json and inspect the emitted Message.java output. Trace how the referenced Context schema is handled after the changes related to issues #1624 and #1296. Done means the referenced Context model is generated consistently with Message.context, without the reported null-pointer failure.