I want to replace the NULL value to “0” in the culumn “<=2000kbps+Count(gNodeB Name)”, but it can...

I want to replace the NULL value to “0” in the culumn “<=2000kbps+Count(gNodeB Name)”, but it can not work, I have tried Rule Engine Node, but below expression cannot work.

$<=2000kbps+Count(gNodeB Name)$ =“” => “0”