Clear NumericStepper Value and get NaN in the display text area

Steps to reproduce:
1. Create Spark NumericStepper
2. Create Spark Button
3. Backspace the value in NumericStepper
4. Click Button

Actual Results:
NaN is displayed in the NumericStepper

Expected Results:
Value remains empty or the default value is set.

Workaround (if any):
Set the callback valueParseFunction to the following function:

private function _numericStepperParse(value:String):Number
{
if (!value)

return Number(value);
}