Fix null query parameter handling in generated RestEasy Reactive clients

Also adds missing query handling tests.

_{The tests for using a `Map<>` as a query param via the `RestQuery` annotation are commented out. This is because I cannot figure out how to get these to work. If I uncomment them then the Quarkus refuses to build the client with an error of `Failed to find converter for java.util.Map`; I am not sure if this is itself a bug or user error.}