[BUG] [Golang] anyOf results in empty Go struct

Description

When Go model code is generated for an "anyOf" type then the result will be an empty struct.

Yaml:

RatType:
anyOf:
- type: string
enum:
- NR
- EUTRA
- WLAN
- VIRTUAL
- type: string

Actual:
type RatType struct { }

Expected:

package openapi
type RatType string

// List of RatType
const (
  NR RatType = "NR"
  EUTRA RatType = "EUTRA"
  WLAN RatType = "WLAN"
  VIRTUAL RatType = "VIRTUAL"
)

openapi-generator version

4.0.0-SNAPSHOT

OpenAPI declaration file content or url

https://github.com/jdegre/5GC_APIs/blob/7298845c7ede07038838d5c40debb083f929f98b/TS29571_CommonData.yaml#L548

https://github.com/jdegre/5GC_APIs/blob/78b04e48a7fb92b1a5dc6f95419c9ed2f6c3c5ed/TS29518_Namf_MT.yaml#L166

Command line used for generation

./run-in-docker.sh generate -i 5GC_APIs/TS29518_Namf_MT.yaml -g go -o /gen/out -Dmodels

Steps to reproduce

./run-in-docker.sh generate -i 5GC_APIs/TS29518_Namf_MT.yaml -g go -o /gen/out -Dmodels

cat out/model_rat_type.go

Reproduce the issue with the provided run-in-docker.sh command and inspect out/model_rat_type.go. Compare the generated result with the RatType declaration and the linked OpenAPI YAML entries. Done means the Go generator no longer emits an empty struct and produces the expected string type and enum constants.