[Fix] take referenced types into account for enum defaults (fixes #14689)

[robbertvanwaveren]

Attempt to resolve a regression in 6.3.0 related to default's for referenced enums.

fixes #14689

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[wing328]

@robbertvanwaveren thanks for the PR. I will see if I can come up with a better fix.

[wing328]

cc @OpenAPITools/generator-core-team

[robbertvanwaveren]

superseded by #14882