When importing Jira issues into Sheets using the Jira Cloud for Google Sheets add-on, empty fields do not return true for the ISBLANK() function in Google.
This can be worked around by using the Len() function against the cell to check if the cell is 0 length, however, Google sheets is seeing the cells as being populated with something.

Adding the value of two cells populated in this way will return 0 or 0 plus any actual number added, so it seems that the value is a null and a zero at the same time.

I would like to suggest potentially formatting null values populated from Jira to blank cells if possible.