fix(properties): correctly handle unknown property get

When parsing invalid documents with [moddle-xml](https://github.com/bpmn-io/moddle-xml) the user occasionally sees

```
"Uncaught TypeError: Cannot read property 'name' of undefined" for Properties.prototype.get
```

This is due to the fact that we did not correctly handle unknown properties in `ModdleElement#get`.

This pull request fixes the behavior.