Fix NullPointerException if exception is in the default package

[mikej]

getPackage() returns null if the class is in the default package so
getPackage().getName() would result in a NullPointerException in
this scenario.

I discovered this when writing some example code for investigating an unrelated issue with Sentry.
It's a bit of an edge case but seems worth adding a fix for it.

[ColinHebert]

I wonder if it wouldn't be better to provide a default value when the package name isn't available, ie (default).

[ColinHebert]

I took the liberty to modify your commit to include the (default) name and add a unit test.

Fixed in @51618d6e0149644b456a5c9df3e3575a035f5d9b

[mikej]

Thanks for the improvements. Having a default value definitely makes sense, and I really like the way you've done the unit test. I was looking for an elegant way to do that.