Default dbt init template uses SQL incompatible with Flink

When users run `dbt init`, the default example model `my_first_dbt_model.sql` contains `select null as id` which fails in Flink SQL with `Illegal use of 'NULL'`. Flink requires `CAST(NULL AS )`.

### Proposed solution

Ship a custom `dbt init` template with Flink-compatible example models, or at minimum document that users should delete the default example models before running `dbt run`.

Start by locating the template used by `dbt init` and inspect `my_first_dbt_model.sql`, then reproduce the failure with `dbt run` under Flink. Determine whether the template can use Flink-compatible SQL or whether the init documentation should tell users to remove the default examples; done means a fresh init no longer fails for this reason, or the limitation is clearly documented.

Written by the indexing model from the issue text.