qbit/core/src/main/java/io/advantageous/qbit/reactive/CallbackBuilder.java
public <T> CallbackBuilder withOptionalCallback(final Class<T> cls, final Callback<Optional<T>> callback) {
        this.callback = callback;
        return this;
    }
public <T> CallbackBuilder withOptionalCallback(final Class<T> cls, final Callback<Optional<T>> callback) {
        this.callback = callback;
        return this;
    }
Builder method to set callback handler that takes an optional string

@param cls      class
@param callback callback
@param <T>      type of callback
@return this