fix: support for nested ref types

[perher]

Fix support for generating go types for schemas using "nested ref definitions", i.e. types that only has allOf, anyOf, oneOf without any properties.

Example schema:

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "type": "object",
  "anyOf": [
    {
      "$ref": "#/definitions/Policy"
    }
  ],
  "definitions": {
    "Policy": {
      "oneOf": [
        {
          "$ref": "#/definitions/Offer"
        }
      ]
    },
    "Offer": {
      "type": "object",
      "properties": {
        "name": {
          "type": "string"
        }
      }
    }
  }
}

without proposed fix the generator would fail with

Could not resolve ref "#/definitions/Policy": cannot resolve reference: expected named type: got *codegen.EmptyInterfaceType
    generation_test.go:261: could not merge anyOf types: types list is empty

also added a new test case to capture this

[codecov]

Codecov Report

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Additional details and impacted files

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  Coverage        ?   42.10%           
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  Files           ?       61           
  Lines           ?     6602           
  Branches        ?        0           
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  Hits            ?     2780           
  Misses          ?     3549           
  Partials        ?      273           

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