[BUG][Go-server] attempt to fix the controller-api mustache template for a nullable bodyParam

[vingarzan]

In the generated code for go-server, in the Controller operation method, a BodyParam is defined as a simple variable. If that is also a Nullable field, the service takes it as a pointer parameter. But the call was made without.

(I tried initially checking just for Nullable, but that broke other things. I'm not sure about the intricate logic here, so would appreciate someone's help to make sure my proposed fix is correct for all cases. For my case of the 5G APIs for Npcf PolicyAuthorization (link is to the bodyParam with the problem) seems to fix the issue.)

Also the examples here seem to be fine now, but I guess that my use case wasn't really covered, since my change does not change any generated code of the examples here. @wing328 I need your help again please and thank you 😜

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