Don't crash system process on empty onTuneFailed

This change:
- modifies internal programSelectorFromHal to return null instead of
  throwing NPE on empty input
- throws NPE on two of its usages where null is unexpected
- *does not* throw NPE on one usage where program selector is optional
  (onTuneFailed in RadioModule.java)

Bug: 147522441
Test: boots, but have no quick way to trigger onTuneFailed(Result::error, {})
Change-Id: I6317da3fbde2bce90079b61bb0839e893e1c4712
(cherry picked from commit dc8a5683a149b4f5ce3d9a9f6198fbe310547dbf)