Adjust number and string variable filters to accept null/undefined

{
  "operator" : "in", // in, "not in", <, >, <=, >=
  "values" : [ null, 1234 ] // null only works for in/not in
}

{
  "operator" : "in", // in, "not in"
  "values" : [ "String 1", "String 2", null ] // null => null/undefined values
}