Adds `if(this.affected(path), ...)`

this.update(function() {
  this.if(this.affected('pdf'), function() {
    this.map('pdf').type(null).optional().writable();
  });
});

(
  "pdf" in request.resource.data.diff(resource.data).affectedKeys() &&
  !("pdf" in request.resource.data.keys())
) ||
(
  !request.resource.data.diff(resource.data).affectedKeys().hasAny([ "pdf" ])
)