Followup b8d8a5b: fix getNativeRange() in null selection case

Invoking getNativeRange( someRange ) should work, even if
the selection is null. Only when the range argument is also
null should a null selection cause it to fail.

b8d8a5b broke this by unconditionally returning null if the
selection was null. This commit restores the original behavior.

Change-Id: I12dfa1d21e75b8d12a71b644b7ce1b4db2abfec5
Merge "Followup b8d8a5b: fix getNativeRange() in null selection case"