Replace (BLANK) by a string on a numeric column

Hello.

I have the following sample table where the hightligted colum stands for number of days that separates us from the target.

Obviously,  this column needs to be numeric, because in my visuals I'm retriveing the MIN value here.

Sometimes, the value here may be (blank). I want to replace this blank value with someting textual, but I can't (and I understand why of course).

My formula here looks like this:

My Formula = 
var _diff = 
IF(
    Table[Col] >= .1,
    DATEDIFF(Table[Date1], Table[Date2], DAY)
)
RETURN
_diff

How can I display a string value when it's blank on a numeric column ?

Thank's a lot.