Handling golang client generation for oneOf with no discriminator and useOneOfDiscriminatorLookup is true

Created by: kandaaaaa

Trying to address issue https://github.com/OpenAPITools/openapi-generator/issues/7961

The idea of the fix is we use the same code of useOneOfDiscriminatorLookup(false) of the condition useOneOfDiscriminatorLookup(true) and discriminator is not defined.

Validation:

1. Have oneOf without discriminator in the openapi spec
2. Use useOneOfDiscriminatorLookup: true to generate golang client
3. Generated code can be compiled and the code will unmarshal each struct

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