The 'empty value' is used to indicate data that is missing - it is present on other rows but deliberately not on this row. The empty value is equal to other empty values, and equal to an empty string (""). The empty value is however not equal to any number.
The 'invalid value' is used to indicate that the value is the result of a calculation gone wrong. For instance, dividing a value by zero will result in an error value. If a function is given wrong arguments or cannot do its work, it will also result in an error value. An error value is never equal to any other value, including other error values.