Bad default value

This default value:

https://github.com/facebookexperimental/siMMMulator/blob/da44d9afdbf3012078deba7a98f4d2999dcccf93/R/step_1_create_baseline.R#L35

(using `my_variables` as the default for `my_variables`) will never work. Either the user specified the argument and the default isn't used, or the user didn't, in which case it will try to use itself as the value, and will fail, as in this SO question: https://stackoverflow.com/q/77511658