Fix support for nullability in Kotlin interfaces and abstract classes

[mwalser]

When encountering a Kotlin property the type parser attempts to find the backing java field: https://github.com/vojtechhabarta/typescript-generator/blob/7a33aa0d8b4ce9e0a7d6626e65195b4c995b2871/typescript-generator-core/src/main/java/cz/habarta/typescript/generator/parser/TypeParser.java#L193

However, in interfaces and abstract classes a property might not have a Java field. In this case the parser falls back to the Java type parser which doesn't know about Kotlin nullability: https://github.com/vojtechhabarta/typescript-generator/blob/7a33aa0d8b4ce9e0a7d6626e65195b4c995b2871/typescript-generator-core/src/main/java/cz/habarta/typescript/generator/parser/TypeParser.java#L200

This PR fixes #708 by changing the behavior to directly use the return type of the Kotlin property.

[johnellinwood]

Any idea when this will get merged?

[yuuzer]

Hello, Our team are waiting for this fix as well.

[wojtre]

Our team is also waiting for this fix, would be cool to merge it.

[mwalser]

@vojtechhabarta Is there anything I can do to support this PR getting merged?

If you have any suggestions about changes I have to make to get this in a mergeable state I would gladly invest some time.

[vojtechhabarta]

@mwalser, @hho thank you for this PR.