any function like ISNULL in FireBird?

This function will return either absolute value, or -1 if the argument was NULL (or not a valid integer). 
function TEST_Abs (val: PChar): integer; stdcall;
begin
  try
    Result := Abs( StrToInt(val) );
  except
    Result := -1;
  end;
end;


DECLARE EXTERNAL FUNCTION TEST_ABS
  CSTRING(20)
  RETURNS INTEGER BY VALUE
  ENTRY_POINT 'TEST_Abs' MODULE_NAME 'UDF_Examples';


SELECT I, TEST_Abs(I) FROM TAB;

       I TEST_ABS
======== ========
       5        5
      -5        5
       0        0
         -1