add basic column tests for arrays and update readme

[ilkernator]

Issue this PR Addresses/Closes

#272

Summary of Changes

Added two new tests to the "column_values_basic" directory and updated the readme-file:

• expect_array_column_to_not_be_empty
• expect_struct_column_to_not_be_empty

Why Do We Need These Changes

For tables that contain columns of type array or array(struct), the values can be sometimes empty without being NULL (e.g. [ ], or [" "]. In this case, the test to expect_column_values_to_not_be_null.sql could be misleading and mask the issue of having empty values. So we would need to make use of Hive functions to assert the array is indeed empty.

Reviewers

@clausherther

[clausherther]

Hi, thanks for this PR.
Before I can take a look at this, we'll need to add integration tests for both macros, which requires adding the appropriately typed columns to the test datasets.
Also, I think I'd prefer implementing this more generically as a single test, ~ expect_column_to_not_be_empty. Would that be possible across all platforms we support?
Also, we'd need the positive equivalent, expect_array_column_to_be_empty.