Add is_null generic data test to test for null values

[james-johnston-thumbtack]

Describe the feature

This generic data test would do the opposite of the not_null generic test. It would assert that the value of a column IS NULL.

It would be particularly useful in conjunction with the where configuration to assert that the value is NULL only under certain circumstances, like if another field is set to a particular value. (Essentially the opposite of what #780 requested.)

Describe alternatives you've considered

The dbt_utils.expression_is_true data test can do it, but it's more verbose.

Additional context

It would be relevant to all database engines.

Who will this benefit?

Here is an example of how the proposed data test could be used. Note in particular the two data tests for picture_image_service_pk.

# columns section of a model:
      - name: media_type
        data_type: string
        data_tests:
          - not_null
          - accepted_values:
              values: ['picture', 'video']
      - name: picture_image_service_pk
        data_type: bigint
        data_tests:
          # Require that an image is present if the media is a picture.
          - not_null:
              config:
                where: "media_type = 'picture'"
          # ----> Demonstration of proposed data test usage is here <----
          # Require that an image is NOT present if the media is not a picture.
          - dbt_utils.is_null:
              config:
                where: "media_type != 'picture'"
          # Alternative example of how you have to do it without the proposed is_null test
          - dbt_utils.expression_is_true:
              expression: "picture_image_service_pk IS NULL"
              config:
                where: "media_type != 'picture'"

Are you interested in contributing this feature?

Sure - if this looks useful, I could contribute a pull request. I don't think I'll need significant assistance to make it, as this seems very straightforward to me.