Fix bug when there are no packages to OTA dexopt.

This would otherwise show up as a "Could not parse version: (all" error
from the otapreopt binary, since `cmd otadexopt next` would return
"(all done)".

Test: OTA update where another bug in OtaDexoptService caused it to
  return an empty list of dexopt commands.
Bug: 251921228
Change-Id: Ie98c9fecc2a57cab7d4d83c369649d7e80929d58