Edge case not properly handled in get_columns_and_types macro

[dlahyani]

This is the problematic excerpt from the macro:

   {%- set relation = adapter.get_relation(
          database=database_name,
          schema=schema_name,
          identifier=table_name) -%}

    {%- set columns_from_relation = adapter.get_columns_in_relation(relation) -%}

This code is problematic in case the requested relation does not exist. In that case adapter.get_relation returns None, so we end up passing None to adapter.get_columns_in_relation which does not expects None.

When using dbt-bigquery adapter it fails with an exception and a cryptic message:

Flow run encountered an exception. 
RuntimeError: Runtime Error
  'NoneType' object has no attribute 'database'

It would be better if before calling to adapter.get_columns_in_relation the macro would check if relation is None and raise an explicit compilation error in that case with a proper error message.