[BUG][Go] UnmarshalJson has unused variable err if spec has oneOf without discriminator

Created by: code-lucidal58

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• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
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Description

In the openAPI spec, if a schema has oneOf but does not have a discriminator and the config useOneOfDiscriminatorLookup is set to True, the unmarshalJson function is incorrectly generated. Such a scenario can arise when the schema looks as follows:

Pet:
  description: a pet
  oneOf:
  - type: 'null'
  - $ref: "#/components/schemas/dog"
  - $ref: "#/components/schemas/cat"

Considering the fact that there are other schemas in this spec that may have oneOf without type: 'null', the configuration useOneOfDiscriminatorLookup: True is required. As a result, the unmarshal function for the above object is as follows:

func (dst *Pet) UnmarshalJSON(data []byte) error {
	var err error
	// this object is nullable so check if the payload is null or empty string
	if string(data) == "" || string(data) == "{}" {
		return nil
	}
	return nil
}

Building this generates an error because the variable err is declared but not used.

openapi-generator version

Master build with last commit id 023e6dd9

Suggest a fix

Allow unmarshalling of data even if nodiscriminator is mentioned whenuseOneOfDiscriminatorLookup: True