Creates a new javafx.beans.binding.BooleanBinding that holds true if the value of a javafx.beans.value.ObservableStringValue is empty.

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

Parameters:

op

the ObservableStringValue

Returns:  the new BooleanBinding

Exceptions:

NullPointerException

if the ObservableStringValue is null

Since:  JavaFX 8.0