I suspect your ProjectCode is a String with no characters, rather than empty. Try changing the test to this
$Minutes/ProjectCode != empty and trim($Minutes/ProjectCode) != ''
Good luck!
Jessica, Robert’s answer is correct, but just so you know, the “trim” function also turns an empty into ‘’ so the following is more concise and works just as well:
trim($Minutes/ProjectCode) != ''
This appears to be Mendix best practice as well, as it’s what any auto-generated code will insert for you when checking if a string is empty.