There isn't a nulltozero expression: there's a built-in function Nz which
will convert a Null to a 0 (or a space) for display purposes, but you don't
use it as a format.
Your options are to create a query that uses the Nz function instead of just
the field in question, or to set the field's format to #,###;-#,###;0;0
assuming it's an integer. If there are decimal points, adjust those formats
accordingly. When you specify 4 separate formats, separated by semi-colons
like that, the first section applies to positive values, the second to
negative values, the third to zeros, and the fourth to Null values.