A DESCRIPTION OF THE PROBLEM :
Mappings on ObservableValues never pass through nulls, even if null is a valid instance of the parameterized type and wouldn't be for the mapped type.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Declare nullable ObservableValue, map it to non-null ObservableValue

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
mapped ObservableValue never contains null
ACTUAL -
null is casted to result type

---------- BEGIN SOURCE ----------
ObservableValue<@Nullable String> input = new SimpleObjectProperty<>(null);
ObservableValue<@NotNull Integer> output = input.map(text -> {
  if (text == null) {
    return 0;
  } else {
    return text.length();
  }
});
assertEquals(0, output.getValue());
---------- END SOURCE ----------