[1.0] Block taking jinja2.runtime.Undefined into DatabricksAdapter.

[ueshin]

Description

Backport of #98

Blocks taking jinja2.runtime.Undefined into DatabricksAdapter.

When calling the adapter method from macros, jinja2.runtime.Undefined can be passed to DatabricksAdapter that could cause unexpected behavior.

For example,

dbt-databricks/dbt/include/databricks/macros/materializations/snapshot.sql

Line 33 in b8152b6

{% if not adapter.check_schema_exists(model.database, model.schema) %}

The model.database is not defined and jinja2.runtime.Undefined will be used to call adapter.check_schema_exists and Cursor.schema() will fail:

[debug] [Thread-13 ]: Databricks adapter: Error while running:
GetSchemas(database=, schema=test16533281379208408401_test_basic)
[debug] [Thread-13 ]: Databricks adapter: <class 'databricks.sql.exc.RequestError'>: Error during request to server
[debug] [Thread-13 ]: finished collecting timing info
[debug] [Thread-13 ]: On snapshot.snapshot_strategy_timestamp.ts_snapshot: Close
[error] [Thread-13 ]: Unhandled error while executing snapshot.snapshot_strategy_timestamp.ts_snapshot
Error during request to server
  Unhandled error while executing snapshot.snapshot_strategy_timestamp.ts_snapshot
Error during request to server

[ueshin]

Thanks! merging.