where a field would go in the query grid, you would put something like this instead:
MyFieldName: IIf([MyFieldName]<=0, "", [MyFieldName])
What this does is says, if MyFieldName is less than or equal to 0, then put a string of zero length into the query, otherwise put the data itself into the query.
Of course you will need to put your own field name(s) into the statement where MyFieldName currently is.