`null` as an example of nullable field

[zlodes]

If I try to set null as an example of some field, it will be ignored.

Example:

Schema::string('foo')
    ->nullable()
    ->example(null),

It will look like this:

"foo": {
    "type": "string",
    "nullable": true
},

But expected:

"foo": {
    "type": "string",
    "nullable": true,
    "example": null
},

It will be correctly rendered using SwaggerUI or Redoc.