On more than one occasion, I needed to delete the data in a single Google Sheet cell. But as noted Common Problems with Sheets article, this is not possible. The only option is to enter some sort of visible character or delete the entire row.
However, I found a simple solution. Just go to emptycharacter.com and click on the Copy To Clipboard in Method 1 and then paste the invisible character into the column of Update Spreadsheet Row action for the cell that you want to blank out. A small red dot should appear, indicating that there is a character there. (The dot may not always appear, but there is a character there.)
Run the zap, and the cell should be empty.