Add logic to handle blank subtask and blank name in names-factory

Before the change, NPE was thrown in case the subtask or name field in
names-factory is blank. This makes the parent task invalid which is not
ideal. Add logic to handle these cases so that only the concerned task
is marked as invalid.

Change-Id: I1f8faf06f7bc55b6619a1ef40bd40ee98bd3b51b