[cpp-ue4] fix for generating formParams in json requests

Created by: leith-bartrich

Issue 10175 describes a potential bug and potential remedy. This pull request is the basic fix I referenced in the issue.

https://github.com/OpenAPITools/openapi-generator/issues/10175

Simply put: if there is no body parameter defined, it tries to generate a json body from formParams. If a formParam isn't required, it uses IsSet from TOptional<> before writing the parameter.

See the referenced issue for the example schema that seems to require this template fix.

To be clear, this isn't really tested against a wide array of OpenAPI schemas. And there are further issues with the generator. But as a first pass, this at least gets a standard Django api-token-auth to be accepted by a Django server, and doesn't ruin other end points in my sample schema.

tagging @Kahncode as requested.

I'd want some more eyes on this at the very least. And my conceptual question from the issue still stands. I presume this to be a template problem, rather than a generator problem. But that might not be correct.

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