Fix crash bug on RefreshTask

Due to Recent root stack change, it have DocumentsInfo but derivedUri
is null. Add NPE check on RefreshTask.

This issue cannot repro on debug rom because only debug rom
will log debugString(), this founction will call deriveFields() and it
will help on generate derivedUri. To avoid such situation, change
debugString implemention.

Fix: 132324406
Test: atest  DocumentsUIGoogleTests
Change-Id: Ic8d185e6f5438fcfc4f52d0b75de305304271bfe