Fix for game init when running under unit test runner

I was trying to run some tests on a class inheriting Game and was encountering NullReferenceExceptions. The class worked fine when being run directly. After some debugging I found this use of Assembly.GetExecutingAssembly() and added a null check.

It turns out this was already fixed in a few other files in:
Unit Test Fixes by tomspilman · Pull Request #1530 · mono/MonoGame