Assuming a field named "MyField", type this into the Field row of your
query:
IsNull([MyField])
and choose Descending in the Sorting row.
Then add MyField in a column to the *right* of that.
How it works:
The calculated field returns True if the field is null, else False.
In Access, True is -1, and False is zero.
Sorting Descending sorts the zero (False) before -1 (True.