Update not_null_proportion.sql to handle function in group_by_columns

[chaubilly]

resolves #

Problem

Refer to In not_null_proportion.sql, when function (e.g. date_trunc) is used in one of group_by_columns, it fails..

Solution

Assign alias to first column in validation CTE, which the alias is used in the following CTE.

Checklist

• [v] This code is associated with an issue which has been triaged and accepted for development.
• [v] I have read the contributing guide and understand what's expected of me
• [v] I have run this code in development and it appears to resolve the stated issue
• [v] This PR includes tests, or tests are not required/relevant for this PR
• I have updated the README.md (if applicable)

[chaubilly]

An error is encountered when running tests against Snowflake. Looks to be a config issue with CircleCI? Can someone help me on this please? Thanks.

10:26:45  Encountered an error:
Runtime Error
  Credentials in profile "integration_tests", target "snowflake" invalid: Runtime Error
    Could not find adapter type snowflake!