[SPARK] Fix NPE for ExternalAppendOnlyMap

It did not handle null keys very gracefully before.

Author: Andrew Or <andrewor14@gmail.com>

Closes #1288 from andrewor14/fix-external and squashes the following commits:

312b8d8 [Andrew Or] Abstract key hash code
ed5adf9 [Andrew Or] Fix NPE for ExternalAppendOnlyMap