fix issue 391 incorrect raise, add tests

[dlawin]

Closes #391

We are inadvertently raising an exception when specifying only prod_database:

Added tests to cover the possible scenarios

Expected (no exception):

dbt_vars_dict = {
            "prod_database": "prod_db",
            "prod_schema": "prod_schema",
            "datasource_id": 1,
        }

dbt_vars_dict = {
            "prod_database": "prod_db",
            "datasource_id": 1,
        }

Exception

dbt_vars_dict = {
            "datasource_id": 1,
        }

dbt_vars_dict = {
            "datasource_id": 1,
            "prod_schema": "prod_schema",
        }