If using an asset-based grid, a value box will display "Undefined" for any asset instance that does not have the corresponding tag. The following script can be used to convert "Undefined" to "-".

public void ValueBox_OnBeginValueChange(object sender, TagSubscriptionArgs subscription, TVQ tvq)
{
  // NOTE: must stop propagation of "ValueFormat" to siblings, otherwise
  // the good value changes will reset siblings when they change.
  // NOTE: any changes to tvq when the IsDefined flag is false will not take
  // affect
  // NOTE: tvq is sometimes null, so we need to account for that
  // NOTE: cannot set control "Value" property to a new tvq while we are in
  // a value change callback or a recursive loop will happen, so axiom
  // prevents that property from being changed in this callback
  // NOTE: "Text" property is used by the server and has no affect when changed
  // by scripting
  string valueFormat = (tvq?.IsDefined == false) ? "-" : "{Value:N2}";
  ControlBase.SetProperty(
    sender,
    propertyName: "ValueFormat",
    propertyValue: valueFormat,
    updateBehavior: UpdateBehavior.NonPropagating);
}