Avoid adding null into UGD opt in list. (#544)
Avoid adding null into UGD opt in list. (#544)

At the beginning of the world there's nothing in the settings global database
and hence when queried updatable_driver_prerelease_opt_in_apps, it returns
null. Appending it back to the opt in list may do no harm but we should avoid
it.

Bug: N/A
Test: `adb shell settings put global updatable_driver_prerelease_opt_in_apps com.android.app` and verify it's appended correctly
Test: `adb shell settings delete global updatable_driver_prerelease_opt_in_apps` and verify no `null` is appended
google/agi,google/agi,google/agi,google/agi,google/agi,google/agi,google/agi,google/agi