[DefaultCodegen] GetReferenceHeader for both calls of headerToCodegenParameter

[Laurens-W]

In #11046 a bug was introduced regarding the fetching of reference headers.
As ModelUtils.getReferencedHeader wasn't called in all cases before headerToCodegenParameter was.
So now it's called inside headerToCodegenParameter as the first thing after null checking the header.

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Possibly solves #11058
Also might relate to #11061

[Laurens-W]

@spacether Since you made the initial feature, can I ask you to review this?

[Laurens-W]

@spacether I've updated the PR

[spacether]

Thank you for your PR. This looks great!