Do not use defaultInstance for persistent repo

In following cases we used default instances to populate the repo:
- If task details for a taskId is not contained in the persisted repo
- If there is a problem reading the persistent repository

These might result in populating the in-memory repo with non-existing
states that are not removed. Instead use null values so we never add any
tasks to repo in case persistent state is not correct.

Bug: 371955621
Test: Manual
Flag: EXEMPT Bugfix
Change-Id: Idbe0663cf143a489ceb596bc0b92690ebb1706ff