String version of numbers can't be used to set values on QListControls.

If you have an item in a list with a value of 245, doing objList->Value = '245' will not work. This is due to a recent change we made where we were trying to differentiate between NULL and 0 values, but we went too far.

This fix equates NULL and empty strings for values, and then equates 0, '0' and false as values separate from null.