Flux endpoint types become nullable in Kotlin

[marcushellberg]

Description of the bug

Kotlin types are not nullable by default. When I expose a Flux endpoint, I do not expect the generated type to be nullable.

Even adding an explicit @Nonnull does not remedy the issue

Minimal reproducible example

See above.

Expected behavior

I expect the TS type to match the Kotlin type, not have an additional | undefined

Actual behavior

See above

Versions:

- Vaadin / Hilla version: 1.2.0.rc1 with multi-module engine
- Node version: 18.8.0
- Java version: 17.0.1
- OS version: 12.4