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

[fivetran-catfritz]

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 for an example.