(fix, go): Don't send 'null' for nil request body

[amckinney]

This updates Go's core.Caller abstraction so that we don't send an explicit null whenever the user sets a nil request type (i.e. for POST requests). Instead, no request body is sent. An additional unit test is included in the generated core_test.go file, too.

user, err := client.Users.Create(ctx, nil) // No request body is sent.

Given that we already use the reflect package for query parameter serialization, this opts to use the simpler reflect solution to determine whether or not an interface value type is set to nil (i.e. via the new isNil helper function).

• If this ever becomes a problem, we can instead update the generated code to include a RequestIsNil sentinel value (because the generated client code can distinguish whether or not the struct type is nil at the core.Caller call-site).