[cpp-ue4] Handled nullable by setting the required tag, effectively making nullables optionals

[Kahncode]

…aking nullables optionals (even if they may be required in the spec). This is because there is no such concept as a value being nullable in C++, and this generator deals with values, not pointers.

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@wing328

[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/8348332465/job/22850142950?pr=18168

i'll update the samples after merging this PR

update: done via 1108ce5