title: Athena query results show null values despite is not null condition in query
body: <p>I have the following query which I run in Athena. I would like to receive all the results that contain a tag in the 'resource_tags_aws_cloudformation_stack_name'. However, when I run the query my results show me rows where the 'resource_tags_aws_cloudformation_stack_name' is empty and I don't know what I am doing wrong.</p>
  
  <pre><code>SELECT 
  cm.line_item_usage_account_id,
           pr.line_of_business,
           cm.resource_tags_aws_cloudformation_stack_name, 
          SUM(CASE WHEN cm.line_item_product_code = 'AmazonEC2'
  THEN line_item_unblended_cost * 0.97    
  ELSE cm.line_item_unblended_cost END) AS discounted_cost, 
  CAST(cm.line_item_usage_start_date AS DATE) AS start_day
  FROM cost_management cm
  JOIN prod_cur_metadata pr ON cm.line_item_usage_account_id = pr.line_item_usage_account_id
  WHERE cm.line_item_usage_account_id IN ('1234504482')
  AND cm.resource_tags_aws_cloudformation_stack_name IS NOT NULL
          AND cm.line_item_usage_start_date
      BETWEEN date '2020-01-01'
          AND date '2020-01-30'
  GROUP BY  cm.line_item_usage_account_id,pr.line_of_business,  cm.resource_tags_aws_cloudformation_stack_name, CAST(cm.line_item_usage_start_date AS DATE), pr.line_of_business
  HAVING sum(cm.line_item_blended_cost) &gt; 0
  ORDER BY  cm.line_item_usage_account_id 
  </code></pre>