String references should be nullable

[willgottschalk]

Describe the bug
The following struct:

type Response struct {
    Field *string `json:"field"
}

generates:

"definitions": {
  "Response": {
    "type": "object",
    "properties": {
      "string": {
        "type": "string"
      }
    }
  }
}

To Reproduce
Steps to reproduce the behavior:

Full Code for the repro

package main

type Response struct {
	Field *string `json:"string"`
}

// @Success 200 {object} Response
func HandleStuff() {
}

Expected behavior
I expect to see either: "type": ["string, "null"] or "required": true field. A reference to a string (or anything) will create a null value when serializing to json

Your swag version
e.g. 1.16.2

Your go version
go version go1.21.1 darwin/arm64

Desktop (please complete the following information):

• OS: mac
• Browser: chrome
• Version: 119

Additional context
Add any other context about the problem here.

[sdghchj]

type Response struct {
    Field *string `json:"field" extensions:"x-nullable"`
}