Fix equality test with ephemeral model + explicit column set

[jtcohen6]

See: dbt-labs/spark-utils#7

This is a:

• bug fix PR with no breaking changes — please ensure the base branch is master
• new functionality — please ensure the base branch is the latest dev/ branch
• a breaking change — please ensure the base branch is the latest dev/ branch

Description & motivation

If equality is comparing an ephemeral model with the compare_columns keyword argument, we want to avoid running adapter.get_columns_in_relation(model) entirely.

Currently, it is the default/fallback arg to kwargs.get(). Instead, we should place it inside a conditional that only runs if kwargs.get(compare_columns) returns none. I was under the impression that these were effectively the same, but this change actually fixes an integration test (test_equal_column_subset) on dbt-utils + spark-utils.

Why did this only break with Spark? Because dbt-spark uses describe table [table_name] to get the columns from a given table, a command that fails if the table does not exist. Most "core" adapters query the information schema instead; if the table doesn't exist, the query returns no results, but it doesn't fail explicitly.

Checklist

• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
  • Spark
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)
• I have added an entry to CHANGELOG.md

[clrcrl]

Makes sense to me!