return default in get_column_values if no execute

[jmriego]

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

Fixes #304
I think get_column_values should return default if not executing.
The issue I'm describing is that this function has a default parameter that will be returned if no data is found. But if DBT is not on execution mode, it will return an empty string
https://github.com/fishtown-analytics/dbt-utils/blob/c8389533cc7f00823c50d08d904cdfa5f714ec8b/macros/sql/get_column_values.sql#L17

Using this simple code will trigger a warning as columns will default to an empty string if no execute:

{% set rep_types = dbt_utils.get_column_values(source_table, 'rep_type', default=[]) %}
{{ dbt_utils.surrogate_key(columns) }} AS row_hash

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
• I have "dispatched" any new macro(s) so non-core adapters can also use them (e.g. the star() source)
• 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

[jasnonaz]

This looks great!! Just adding one additional bit of logic as in the case someone doesn't specify default, we'd want to return an empty list rather than none.

[joellabes]

Thanks for the contribution @jmriego!