WHERE COALESCE(mycolumn, '') = ''
Read the documentation: COALESCE()
Or the other way around; convert empty strings to NULL;
WHERE NULLIF(mycolumn, '') IS NULL
Of those two, I would prefer COALESCE() as it is part of the ANSI SQL standard
You can experiment with it yourself, just do this;
SELECT mycolumn AS orig_value, COALESCE(mycolumn, '') AS coalesce_value, (COALESCE(mycolumn, '') = '') AS compare_result FROM mytable;
This will show the original value, the 'coalesce' value and the result of the comparison side by side for every row in the table