Fix generation of null type for Option<T>

[DanielleHuisman]

Schema

#[derive(ToSchema)]
struct Top {
    bounds: Bounds<i32>,
}

#[derive(ToSchema)]
struct Bounds<T> {
    min: Option<T>,
}

Current behaviour
The Bounds_i32 schema lists min as a non-nullable integer.

{
  "components": {
    "schemas": {
      "Bounds_i32": {
        "type": "object",
        "properties": {
          "min": {
            "type": "integer",
            "format": "int32"
          }
        }
      },
      "Top": {
        "type": "object",
        "required": [
          "bounds"
        ],
        "properties": {
          "bounds": {
            "$ref": "#/components/schemas/Bounds_i32"
          }
        }
      }
    }
  }
}

Expected behaviour
The Bounds_i32 schema should list min as a nullable integer.

{
  "components": {
    "schemas": {
      "Bounds_i32": {
        "type": "object",
        "properties": {
          "min": {
            "oneOf": [
              {
                "type": "null"
              },
              {
                "type": "integer",
                "format": "int32"
              }
            ]
          }
        }
      },
      "Top": {
        "type": "object",
        "required": [
          "bounds"
        ],
        "properties": {
          "bounds": {
            "$ref": "#/components/schemas/Bounds_i32"
          }
        }
      }
    }
  }
}