Mmm, interesting question!
As far as my current use case - the value should never be empty, in that this attribute is set when a user account is created, or when it has been renewed (the attribute is to carry the number of days past the annual review date the account is, and is reset to 0 when successfully renewed).
However, this does touch on some of the logic I'm actually using this for, in that I'm trying to catch any condition that my workflows may encounter due to unforseen circumstances.
Core example is, if my workflows expect an 'account review cycle' to be triggered on the review due day, but we had a power-down on that day so the daily scheduled task didn't execute, what happens on the next day? If we allow our job to trigger on past dates to 'catch up' there is a risk that we may re-execute the job in error, so I'm looking at this attribute to be a 'safety check' i.e. "IF Review date in past AND Counter > 0 THEN "Review Task" has already initiated so Cancel".
We can also use this to trigger specific reminders i.e. send gentle email after 7 and 14 days, but start getting "shouty" after 21, and scream on days 26, 27 and 28 without constantly performing date checks (or making the date comparison another safety check)!
So, in this case, I think we can assume empty is == 0, but something else to ponder for me!
Rgds