[FEATURE] - Update fill_pass_through_columns macro so pre-defining variables not required

Currently, `fill_pass_through_columns` fails if the passthrough variable is not defined in the project yml. In order for us to eliminate the need to pre-define these variables in our packages, this should be updated so a default value is declared if the variable does not exist. See [persist_pass_through_columns](https://github.com/fivetran/dbt_fivetran_utils/blob/releases/v0.4.latest/macros/persist_pass_through_columns.sql) for an example.