Solve a possible NPE in the Documents provider

This commit follows an internal bug report to fix a NPE that would be
thrown in specific situations on specific devices. All callers should
be able to handle the returned null value but if they don't, we can
worry about that later down the line. This null value should not be
returned under normal circumstances anyway.

Change-Id: I274844bf07733643923d5729259f1d9d9cf75e54
Merge branch 'kitkat' of https://github.com/vedantgp/android_frameworks_base into kitkat