Hi, I believe I had the same problem with a text field in the table. I was able to make my query from the table using the "Select by Attribute" tool at the top of the table. In the Select by Attribute dialog I selected "Add Clause", and in that dialog selected my Attribute Field, the comparison value ("is equal to", "is not equal to", etc.), then click on the little slider button marked SQL. In the SQL dialog that comes up, the Clause comes in built except for the value you want to use. In this case where there is no value, I used two single quotes (ex. '' ,with no space). This selected the empty values in the field I was querying.
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