Test that we correctly back up an empty file.

(It turns out that we do.  It didn't used to work, I'm not
sure what changed)
diff --git a/tests/backup/test_backup.sh b/tests/backup/test_backup.sh
index f6c73d8..6ef5dff 100755
--- a/tests/backup/test_backup.sh
+++ b/tests/backup/test_backup.sh
@@ -12,6 +12,7 @@
            echo -n first file > /data/data/com.android.backuptest/files/file.txt ; \
            echo -n asdf > /data/data/com.android.backuptest/files/another_file.txt ; \
            echo -n 3 > /data/data/com.android.backuptest/files/3.txt ; \
+           echo -n "" > /data/data/com.android.backuptest/files/empty.txt ; \
 "
 
 # say that the data has changed