Fix dynamic models with contract enabled

[carolinabtt]

resolves #1002

Problem

When a model is Contract Enforced and the sql statement contains the _dbt_max_partition variable, dbt does not declare the variable before creating the cte.

This results in the following error:

• Unrecognized name: _dbt_max_partition at ..

Solution

This PR fixes the bigquery__get_empty_subquery_sql macro, by checking 2 conditions:

1. if the model is contact enforced
2. if the model contains _dbt_max_partition variable

If the conditions are met, the _dbt_max_partition is declared.