code: public Optional<String> getSlotValue(String slotName) {
          return getSlot(slotName).map(Slot::getValue);
      }
docs: Returns the value from the given {@link Slot} in the request.
  
  This method attempts to retrieve the requested {@link Slot}'s value from the incoming request. If the slot does
  exist in the request, an {@link Optional} empty is returned.
  
  This method returns an {@link IllegalArgumentException} if the incoming request is not an {@link IntentRequest}.
  
  @param slotName name of the slot to retrieve
  @return an {@link Optional} containing the target slot's value if it exists in the request, else an empty {@link Optional}