Fix a NRE in Mouse.GetState() when used to initialize a static field.

A project I'm porting from XNA to MonoGame initializes a static field using Mouse.GetState(). The fix returns a default MouseState if Mouse.PrimaryWindow hasn't been initialized. A more correct implementation would be to forward to a platform-specific (or GameWindow-specific) static method that decided whether to poll hardware (XNA queries the mouse directly, always) or return a default state if hardware isn't ready yet.