Gallery2: fix the bug that only delete parts of photos when select all.

DataManager.getMediaObject() may return null value, so DataManager.delete()
and DataManager.getSupportedOperations() will be interrupted
and only parts of photos were deleted.

Add null value check, ensure the DataManager.delete() and
DataManager.getSupportedOperations() can execute normally.

Change-Id: If5a4f1118d9a03d9d70c2e237347d20b3142b031
CRs-Fixed: 954040