We have a Zap that populates Google Sheets with values. Some of the values are pulled from Lookup Tables. There are scenarios when we want the Zap to overwrite an existing value in the Google Sheet with NULL.
Can a Lookup Table offer NULL as an output value?
Asking the Zapier team about this as promised - will get the answer here so we can all benefit.
So, while you can't use Zapier to generate a null value to send to Google Sheets - you can achieve what you want to do using a workaround involving Google Scripts.
Here are the details: https://webapps.stackexchange.com/questions/106051/clear-cell-based-on-modification-of-another-cell
Essentially write a value to a designated other cell, which will then trigger a script that will clear the cell you want empty.
Let us know if and how well that works out for you.