[php-symfony] fix handling of endpoints with "text/plain" or "image/png" response type

[gturri]

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This fixes #21256 (and this subsequently fixes #13334, which is a subset of the other one).

This PR implements the 3 fixes that are required to be able to handle all the endpoints described in this OpenApi specification:

openapi: 3.0.3
info:
  title: repro issue openapi-generator
  version: 0.0.1
paths:
  /get-bool:
    get:
      summary: get a boolean
      responses:
        "200":
          description: OK
          content:
            text/plain:
              schema:
                type: boolean
  /get-image:
    get:
      summary: get an image
      responses:
        "200":
          description: OK
          content:
            image/png:
              schema:
                type: string
                format: binary
  /get-string:
    get:
      summary: get a string
      responses:
        "200":
          description: OK
          content:
            text/plain:
              schema:
                type: string
  /get-int:
    get:
      summary: get an int
      responses:
        "200":
          description: OK
          content:
            text/plain:
              schema:
                type: integer
  /get-number:
    get:
      summary: get a number
      responses:
        "200":
          description: OK
          content:
            text/plain:
              schema:
                type: number

Namely this:

• ensures the generated method Controller.getOutputFormat returns a valid format when the user accepts */*
• does not try to call ->serialize($data, ...)`` when the 2nd argument is null` (it would crash (error 500)). Instead it uses a straightforward way to handle the serialization in those simple cases
• define relevant return type when the specification declares response with context text/plain

(nb: to make it easier to review I did a separate commit for each of those 3 fixes)

Ping technical committed, ie: @jebentier @dkarlovi @mandrean @jfastnacht @ybelenko @renepardon (and thanks for your work!)

[gturri]

I'm afraid I'm seeing an issue in the last commit that I did not spot yesterday. I'm closing that PR and will recreate a new one once I have more thoroughly tested it.
Sorry about that.