R Custom Power BI Visualization Variable Input Handling

I am trying to create a custom viz (scatter plot) with the ability to provide different options for colour and shape of the points using ggplot2.

My visualisation works fine when I specify both the Shape and Colour parameter, however, I want to be able to only supply one or the other down the road, not always both.

This is what my parameter options look like:

While my R script.r file for ingesting data is:

This is the error I get when I don't include Colour or Shape or both.

I feel that the ability to handle not supplying an argument should be handled within the script which I have tried to do. However, I am now stuck.