Conversion of a symbol to a word.

Hi all! Just wondering if I can get some direction on how to solve this issue. I need to convert rows with underscores into the word “Null” (not a missing value).

I have tried many different ways to try and solve this, and this is the closest I have gotten. However, it changes ALL the rows into “Null”, which is not what I am after.

Of note, one of the groups in the row has a singular underscore. So I assume I will need to use a rule that only changes the row if it starts with an underscore?