Gradebook: How do I specify "blank" or "no grade" as the result of a formula?

I'm using the "formula" style of gradebook template, because none of the others offer the flexibility I need. Getting Brightspace to handle blank grades reasonably with this system has not worked out so far. The "don't include blanks in averages" setting does not appear to apply to formulas, which tend to throw an error if a blank item (e.g., numeric grade with no grade, yet) is included.

One potential fix for this would be for the formula editor to allow a blank numerical value (or NA or NULL or empty, etc.) as the result of a formula. For example, you can do this in Excel:

=if(isblank(grade_1), "", grade_1)

meaning that if grade_1 has not been assigned yet, then have the formula return a blank/null value; otherwise, return the grade_1 value.

For some purposes I have been successful in the first part of that example (testing to see if grade_1 has any value assigned) using ">0" that is, if BS sees a blank, that sometimes appears to count as "not greater than zero", so I can get the formula to return a different value for a grade not yet assigned.

The second part of the formula is giving me fits. How do I specify that, if <condition> applies, the formula should return a Blank or Null or N/A or empty value? Is there a special symbol for this, perhaps? I've tried leaving that argument blank (error) and specifying "" (two double quotes; error) or '' (two single quotes; error).

Any ideas? Or any ideas for how to do what I'm trying to do in a different way?