PostgreSQL find blank rows

You need to use AND to check _http_requests.currentsheet. If it was NULL, then it would always be true for the <> '' check and vice versa.

As a way simpler example, you can use select statements without a table to help debug this sort of thing (from psql or whatever SQL query tool you like):

select (''   is not null or ''   <> '') as empty_result,
       (null is not null or null <> '') as null_result;

 empty_result | null_result 
--------------+-------------
 t            | 

If the string is '', you get true. If the string is null, you get null (this is because comparisons with null are SQL oddities -- select null = null; results in null). Let's see what happens when we replace or with and:

select (''   is not null and ''   <> '') as empty_result,
       (null is not null and null <> '') as null_result;

 empty_result | null_result 
--------------+-------------
 f            | f

Neat! With X is not null and X <> '', we get false when X is either '' or null.

So the way to phrase the select statement to do what you actually want is:

SELECT *
  FROM PUBLIC._http_requests
  WHERE _http_requests.currentsheet IS NOT NULL
    AND _http_requests.currentsheet <> ''
    AND _http_requests.session_id IS NOT NULL
    AND _http_requests.http_referer IS NOT NULL;

I think you just need AND _http_requests.currentsheet <> ''. The AND is important there so that we exclude both.