Have `Prefs.notificationdata.firstVersion` default to "0" instead of undefined
Background
With issue 7168 we added core code to add a "firstVersion" GET parameter to the notification downloads. For that to work, the Prefs.notificationdata.firstVersion preference was expected to start as "0". Unfortunately, since that wasn't mentioned in the integration notes we didn't know to update the adblockpluschrome code to give that preference the correct default value. This meant that it continued to default as undefined instead. The result was the code to add the "firstVersion" parameter effectively didn't do anything. Luckily this was caught by Ross in the pre 3.6 release testing.
What to change
Ensure that Prefs.notificationdata.firstVersion defaults to "0" instead of undefined.
Notes
Hints for teseters
See the comprehensive testing instructions given in the corresponding adblockpluscore issue.