[kotlin-spring] Optional delegate in ApiController constructor

[mCzolko]

If the delegate is optional, put it as optional in constructor

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[mCzolko]

@jimschubert (2017/09) ❤️, @dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03)

[Zomzog]

LGTM

[wing328]

@mCzolko sorry for the delay. Thanks for the PR, which has been merged into the master