You can check if a cell is blank by simply referencing the cell you want to check in the logic part of the IF statement. Here is the formula I would use:
While @Joseph_Abraham is correct, there are a couple of other things to point out:
If you want the field to remain empty if the condition in an IF() function is false, you don’t need to use BLANK() as the final argument. You can leave that off and Airtable will automatically leave the field empty.
When checking a number field, a 0 (zero) in that field will also be treated as equivalent to false when using the field-name-only test that was described. In those cases, a suggested workaround is to concatenate the number field output with an empty string, which will force the result to a string. In that case “0” is a non-empty string, which is equivalent to true. A truly empty number field concatenated with an empty string will leave an empty string, which is equivalent to false.
With those concepts applied, the formula becomes this:
Thanks @Justin_Barrett! TIL that a zero will also be treated as false when using the field-name-only test. I will keep your workaround in mind next time!