Invalid value error with dbt test --store-failures

[jtcohen6]

Compilation Error in test not_null_model_a_id (models/schema.yml)
  Invalid value provided for 'persist_docs'. Expected dict but got value: None

  > in macro comment_clause (macros/adapters.sql)
  > called by macro spark__create_table_as (macros/adapters.sql)
  > called by macro create_table_as (macros/adapters/common.sql)
  > called by macro statement (macros/core.sql)
  > called by macro materialization_test_default (macros/materializations/test.sql)
  > called by test not_null_model_a_id (models/schema.yml)

The issue is with how we've implemented persist_docs on Spark, which is subtly different from other databases. It's called right within the create table as statement, which we use for tests. If persist_docs isn't a dictionary, we raise a compilation error... but we probably shouldn't if it's None.

I think this will be a simple fix.