Where `isOptional` is used, also check for `isNullable` i.e.
values are not considered required if they are Kotlin nullables:
- spring-messaging: named value method arguments
- spring-web: named value method arguments
- spring-webmvc: request parts
This means that Kotlin client code no longer has to explicity specify
"required=false" for Kotlin nullables -- this information is inferred
automatically by the framework.
Issue: SPR-14165
fada91e538