dataset columns types : null vs "null"

Question

I have a query on a myqsl server returning a columns with numbers and nulls.

I define a DataSet on the KA server.
When I link the DataSet in RportDesigner, the column is created as String.
The null values, are real string containg characters "null".

Testing in javascript:
if( myValue == null )
is evaluated as false.

if( myValue == "null" )
is evaluated as true

if I cast this column to Number in the report, the conversion from "null" to BigBecimal fails.

Where can I define the correct datatypes on the server DataSet ?

I can't create a simple report and I can't belive this.