Edit undefined values

**Not sure if this is considered a bug or a feature request**

Aurelia inspector allows you to modify model values by double-clicking them.

But it doesn't work for values that have not been set yet (`undefined`). In this case double-clicking currently does nothing, no text box appears.

No files, tests, or entry points are named. Reproduce the issue in the Aurelia inspector with a model property whose value is undefined, then trace the double-click editing flow. Done means double-clicking an undefined value opens a text box and allows the value to be modified.

Written by the indexing model from the issue text.