Fix parsing no content into null instead of 'null'

[olivernybroe]

Hello 👋
Thank you for the package, we are really enjoying all the work you put into it. I hope you will accept this small change which we think might be a bug :)

Problem

When having a response with no content in it, it ends up showing as plaintext in openapi spec.

#[Response(status: 404, description: 'Organization not found')]

Will be generated as

        404:
          description: 'Organization not found'
          content:
            text/plain:
              schema:
                type: string
                example: 'null'

solution

To fix this problem, we make sure that if the content is null, we pass null forward when converting the attribute to an array. Previously the null value was passed through json_encode which will convert it into 'null'.

After this solution, we end up with

        404:
          description: 'Organization not found'
          content:
            application/json:
              schema:
                type: object
                nullable: true

[shalvah]

Good catch, thanks!