Missing @Nullable annotation on GraphiQlHandler

[mhalbritter]

Hello team 👋,

https://github.com/spring-projects/spring-graphql/blob/main/spring-graphql/src/main/java/org/springframework/graphql/server/webflux/GraphiQlHandler.java#L53

It says in the JavaDoc:

graphQlWsPath optional path to the GraphQL WebSocket endpoint

But graphQlWsPath is not marked as @Nullable.

This affects both the webmvc and the webflux implementation.