DAX help - Custom column should be null if a value in another column is null

I have created a custom column [Part] which is concatentation of two other columns [Customer OTI number] & [Order Part Number]. Both of these columns are of type text.

I need the value in the new custom column [Part] to be blank if the value in [Order Part Number] is blank.

Currently it looks like this below. I've highlighted the problem in yellow. I need these values to be blank.

Any ideas how best to do it?