Change your expression to be:
If (IsEmpty ( Safety Department Signature ) ; "Unreviewed" ; "Reviewed")
is a Boolean expression that compares the value of Status to the value of "Unreviewed". If the values are equal, it evaluates as True or the number value 1. If the values are not equal, the result of False returns a 0.
Wow... Thanks so much! Would have overlooked that for hours!