Don't attempt to work on empty pending starts list

Also plug a bookkeeping hole where information about pending service
starts was cleared but delayed starts relying on that information were
accidentally left dangling.

Fixes: 124467582
Fixes: 124399808
Test: apct/device_boot_health_check_extra
Change-Id: Ic1fec244c76be0969860fd5abee31be575ce0eca