Add NULL as reserved keyword cpp

[martonmiklos]

I came across schemas containing NULL as enum item resulting un-compile-able code.

Dependant on #17802

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@ravinikam @stkrwork @etherealjoy @MartinDelille @muttleyxd

[wing328]

when you've time, can you please PM me via Slack to quickly walk through your PRs? Thanks

https://join.slack.com/t/openapi-generator/shared_invite/zt-12jxxd7p2-XUeQM~4pzsU9x~eGLQqX2g

[martonmiklos]

when you've time, can you please PM me via Slack to quickly walk through your PRs? Thanks

https://join.slack.com/t/openapi-generator/shared_invite/zt-12jxxd7p2-XUeQM~4pzsU9x~eGLQqX2g

Sure, which timezone are you located?

[wing328]

+0800 (Hong Kong)