Ensure keyboard is not NULL before sending enter notification

[flaviozavan]

Ensures keyboard notification enter events are only sent to valid keyboards. Null values could be seen when focus was changed using the mouse after unplugging a keyboard.

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To test:

1. Use keyboard;
2. Open 2 views;
3. Focus one of them;
4. Unplug keyboard that was last used;
5. Click the other view;
6. Watch segfault before this change, or no segfault with it.

[inclement]

Thanks - seeing the change makes me feel certain there are other places where unexpected combinations of input devices aren't handled correctly, but that's a separate problem.