Update jsTree.directive.js

Checking if new value of tree model is defined so that the tree init is not called twice (when watcher is initialized and when watched value is changed).
Added check if new tree data is defined.

This needs to be checked so the tree init is called only once. Otherwise "Uncaught TypeError: Cannot read property 'triggerHandler' of null " is shown in console, because the init is called twice (once when initialising watcher and second time when the watched value changes)
Merge pull request #7 from magdajwer/master

Checking