Fixes isFreeFormObject detection

[spacether]

We have a bug in master branch where this schema:

    A:
      type: object
      additionalProperties:
        $ref: '#/components/schemas/B'

Is incorrectly interpreted as FreeFormObject when it should be a MapSchema
Per our java documentation a schema is FreeForm if:

A free form object is an object (i.e. 'type: object' in a OAS document) that:
1) Does not define properties, and
2) Is not a composed schema (no anyOf, oneOf, allOf), and
3) additionalproperties is not defined, or additionalproperties: true, or additionalproperties: {}.

Schema A does not satisfy requirement 3. Requirement 3 only allows additionalproperties values of null, true, and empty object {}. We have {$ref: '#/components/schemas/B'} which is different.

This PR fixes that bug by making sure that when we are in isFreeFormObject we check to see that a ref does not exist in additionalProperties.

• A test has been added demonstrating that the above model is generated with our updated code

• If merged, this PR will fix [BUG] python-experimental broken on master (java.lang.NullPointerException) #7372 (comment)

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[mks-m]

if i understand correctly, if the error is in modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java then it should be affecting all languages. why did i only get the exception in python-experimental?

[mks-m]

otherwise the fix looks good, thanks for addressing this so quickly!

[spacether]

if i understand correctly, if the error is in modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java then it should be affecting all languages. why did i only get the exception in python-experimental?

So there are two issues at play with why the error is seen in python-experimental and not other generators.

1. Is schema A seen as a FreeFormObject? In master branch yes it is incorrectly classified as FreeFormObject rather than MapSchema. That means that it will not be generated.

• [all generators impacted]

2. python-experimental post processes all models to make sure that their imports are formatted correctly for python. Because schema A was not generated, this line throws a null pointer exception. This only happens in python-experimental because we are post processing all models (whether or not they are generated), and we hit some data where a value that we are trying to grab is null. If other generators were trying to get the "models" value in the schema A model they will also hit this exception. They don't do this same iteration, or they handle the null condition.

• [only python-experimental impacted]

That's why it is only seen in python-experimental.

[wing328]

LGTM