[OCaml] Fix default optional list

[sir4ur0n]

Add a default value for non-required lists.
Also, indent the model template to make it easier to read and maintain.

I have kept 2 commits to easily review the cleanup vs the actual fix, and its impact on the sample petstore.

Closes #20777

@cgensoul

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[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[sir4ur0n]

Ah thanks, we mostly use Gitlab at Routine so I had not added my Routine email to my Github profile.

Fixed now 👍

[cgensoul]

I quickly reviewed both commits and everything looks good to me. It's a pity that the generated code looks uglier but I admit that writing the template on a single line to keep the output pretty was a real pain.

Thanks for your contributions to the OCaml generator ! I am glad someone uses it :)

[sir4ur0n]

If it were up to me I would ocamlformat on the flight each file before writing it, in the generator itself 😄
But in any case I call the formatter in my script that generates the code in my own repo so I'm not affected

[MisterDA]

FYI Jane Street started their own openapi 3 generator.
https://github.com/janestreet/ocaml_openapi_generator