How to block "null" value inserted by REST API calls?

The client performs bulk activity creation from REST API calls.

However, some Cell values were filled with "null 99999999999".

We need to substitute the value for "99999999999", without the "null".

How to do this?

I tried to add the Regular expression "/[0-9]/", but it doesn't work for values entered by REST API calls, only for values entered in web/mobile application.