completions_tree_view.getCellProperties: fix for XULRunner >= 22

Conkeror would issue errors like the following to the terminal when
running on XULRunner 22 or later:

    Console error: [JavaScript Error: "TypeError: props is undefined"
      {file: "chrome://conkeror/content/minibuffer-read.js" line: 69}]
      Category: chrome javascript

The behavior of getCellProperties was changed in XULRunner 22, so this
patch is an update for that change.