Allow to use copy trigger without expression

[andrewtelnov]

The current JSON doesn't work:

{
    elements: [
      { type: "text", name: "q1" },
      { type: "text", name: "q2" },
   ],
    "triggers": [
      {
        "type": "copyvalue",
        "fromName": "q1",
        "setToName": "q2"
      }
    ],
  }

Our triggers doesn't work without expression. The current JSON on changing "q1" value generates exception.
We can support this case, if expression is empty then check if "q1" (fromName") is changed.