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

[kandaaaaa]

Trying to address issue #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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@ph4r5h4d

[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[wing328]

Please also merge/rebase the latest master into your branch/repo as most of the changes (sample update) have been already done in the master of the official branch.

[kandaaaaa]

@wing328 done, please review

[wing328]

Tested locally and the result is good. Thanks for the PR.