fix(athena): handle unpartitioned models in create_table_as_with_partitions

[dtaniwaki]

resolves #1742
docs N/A

Thank you for maintaining this project! I'd appreciate your review on this bug fix for the Athena adapter.

Problem

When a model with no partitioned_by config reads from a highly-partitioned Hive source table, Athena may return TOO_MANY_OPEN_PARTITIONS due to the source table's partition count. safe_create_table_as falls back to create_table_as_with_partitions, which unconditionally calls get_partition_batches. This calls adapter.format_partition_keys(None) and crashes with TypeError: 'NoneType' object is not iterable.

Compilation Error in model my_model (models/my_model.sql)
  'NoneType' object is not iterable
  > in macro create_table_as_with_partitions (macros/materializations/models/table/create_table_as.sql)
  > called by macro safe_create_table_as (macros/materializations/models/table/create_table_as.sql)

This affects both temporary=False (full-refresh) and temporary=True (incremental merge's __dbt_tmp), the latter of which regressed after the temporary branch unification in #1711.

Solution

In create_table_as_with_partitions, check partitioned_by before calling get_partition_batches. If the model is unpartitioned, skip batch INSERT and create the target table directly from the staging table via simple CTAS. The staging table is created with skip_partitioning=True, so it has no partitions and the CTAS from it will not trigger TOO_MANY_OPEN_PARTITIONS.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX