Fix default value generation for composed schemas in AbstractJavaCodegen

Created by: cowclaw

When using composed schemas, null is generated as a default value, even if no default value is given. This bug is resides in AbstractJavaCodegen and is thus present in many generators extending the AbstractJavaCodegen.

Samples showing the buggy behavior already exist in bin/configs/java-native-openapi3.yaml and bin/configs/java-jersey2-8.yaml using modules/openapi-generator/src/test/resources/3_0/java/petstore-with-fake-endpoints-models-for-testing-with-http-signature.yaml. Below is an excerpt of this yaml, showing the definition of the composed schemas.

Drawing:
  type: object
  properties:
    mainShape:
      # A property whose value is a 'oneOf' type, and the type is referenced instead
      # of being defined inline. The value cannot be null.
      $ref: '#/components/schemas/Shape'
    shapeOrNull:
      # A property whose value is a 'oneOf' type, and the type is referenced instead
      # of being defined inline. The value may be null because ShapeOrNull has 'null'
      # type as a child schema of 'oneOf'.
      $ref: '#/components/schemas/ShapeOrNull'
    nullableShape:
      # A property whose value is a 'oneOf' type, and the type is referenced instead
      # of being defined inline. The value may be null because NullableShape has the
      # 'nullable: true' attribute. For this specific scenario this is exactly the
      # same thing as 'shapeOrNull'.
      $ref: '#/components/schemas/NullableShape'
    shapes:
      type: array
      items:
        $ref: '#/components/schemas/Shape'
  additionalProperties:
    # Here the additional properties are specified using a referenced schema.
    # This is just to validate the generated code works when using $ref
    # under 'additionalProperties'.
    $ref: '#/components/schemas/fruit'
Shape:
  oneOf:
    - $ref: '#/components/schemas/Triangle'
    - $ref: '#/components/schemas/Quadrilateral'
  discriminator:
    propertyName: shapeType
ShapeOrNull:
  description: The value may be a shape or the 'null' value.
    This is introduced in OAS schema >= 3.1.
  oneOf:
    - type: 'null'
    - $ref: '#/components/schemas/Triangle'
    - $ref: '#/components/schemas/Quadrilateral'
  discriminator:
    propertyName: shapeType
NullableShape:
  description: The value may be a shape or the 'null' value.
    The 'nullable' attribute was introduced in OAS schema >= 3.0
    and has been deprecated in OAS schema >= 3.1.
  oneOf:
    - $ref: '#/components/schemas/Triangle'
    - $ref: '#/components/schemas/Quadrilateral'
  discriminator:
    propertyName: shapeType
  nullable: true

As the commit shows, after re-creating the samples, the default null values are gone.

cc: @wing328 @bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @nmuesch

Thanks in advance!

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