Creates a new javafx.beans.binding.BooleanBinding that holds true if the values of two instances of javafx.beans.value.ObservableStringValue are not equal.

Note: In this comparison a String that is null is considered equal to an empty String.

Parameters:

op1		the first operand
op2		the second operand

Returns:  the new BooleanBinding

Exceptions:

NullPointerException

if one of the operands is null