Handle undefined text when highlighting

It handles the situation where you have:
<code>o = { name: "Foo" }</code>
But the template looks like this:
<code>
...
&lt;div&gt; ng-bind-html="o.name | highlight:term"&lt;/div&gt;
&lt;div&gt; ng-bind-html="o.description | highlight:term"&lt;/div&gt;
...
</code>