Fix an NPE when FileCollector runs.

In some cases, the path for a VolumeInfo may be null. The previous
implementation assumed it was always non-null, however. This backs out
of the calculation early if we cannot find the files.

Change-Id: Icfe265be738c5ccb16917ef339b2b872f2057f07
Fixes: 38308873
Test: No observed phone crashes with a flashed build.