Default value of `date` QML primitive?

Default value for date properties is not specified in documentation. Apparently, one can not rely on null comparison to detect uninitialized date properties, and an attempt to print such value to the console results in this:

property date lastEvent
if (lastEvent !== null) {
  print(`E: lastEvent`)
}

qml: E: Invalid Date

1. What the heck? I've found nothing like that on MDN.
2. OK, so how do I check whether a date property has been initialized?