[java] fixed pojo template for nullable fields (!12630) · Merge requests · OpenAPI Tools / openapi-generator

Created by: valmoz

Related to https://github.com/OpenAPITools/openapi-generator/issues/12549 this PR modifies the template to be able to manage also JSON fields explicitly set to null (not only omitted field case).

It uses Gson's "isJsonNull" method to check this case, instead of only checking if the key isn't present.

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