Input Control default of [Null]

I upgrade 5.0.1 to 5.6.

In new version all  "Single Value" input control, not mandatory with datatype of Text, when used it always displays a default of "[Null]".

In old version the input control was correctly empty.

Therefore when the control is used, the user always needs to delete the "[Null]" text or jasper take "[Null]" as value of the parameter.

The only solution I found is set the default value as "" (empty string).

But haveI to change all my input control after upgrade?!?

With more of 250 reports it's a problem..............