I'm filling in some missing attributes in a feature layer using 'calculate field' and I tried:

and this works, kind of. Empty fields gets filled in with '3' but any field already filled are emptied and <null> afterwards.
So for the moment I have:

which covers all possible values (in the coded value domain on this attribute). This works but is cumbersome.
What I would like to do is something like

How can I achieve this ... else do nothing ... ?
Thanks in advance for your time.
Bert Kraan