fix(python-fastapi): remove 200 fallback code and use default (#12481)

[sickinga]

Currently the default code of the Python FastAPI would be 200 when in reality it would be more suitable to keep it empty (or 0), as stated in #12481.

Here is the OpenAPI.yaml that was used as input:
https://github.com/OAI/OpenAPI-Specification/blob/aa91a19c43f8a12c02efa42d64794e396473f3b1/examples/v3.0/petstore-expanded.yaml#L51

Here is what it currently produces:

    responses={
        200: {"model": Pet, "description": "pet response"},
        200: {"model": Error, "description": "unexpected error"},
    },

this is what it now produces:

    responses={
        200: {"model": Pet, "description": "pet response"},
        "default": {"model": Error, "description": "unexpected error"},
    },

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  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
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@cbornet, @tomplus, @krjakbrjak, @fa0311, @multani

[sickinga]

@wing328 should be ready for review, right? :)

[renatomameli]

Thank you @sickinga. We really rely on this fix and it would be great if we can get this in an upcoming release! @wing328

[wing328]

PR merged. Thanks for the contribution