factory: Fix inconsistent-return-statements pylint error

BUG=chromium:999876
TEST=make lint

Change-Id: I59595c45cffa253191f99511385f24aaf7d6ac68
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/factory/+/2006749
Commit-Queue: Yilin Yang (kerker) <kerker@chromium.org>
Tested-by: Yilin Yang (kerker) <kerker@chromium.org>
Auto-Submit: Yilin Yang (kerker) <kerker@chromium.org>
Reviewed-by: Cheng Yueh <cyueh@chromium.org>
diff --git a/py/utils/pygpt.py b/py/utils/pygpt.py
index 38d7679..c49faac 100755
--- a/py/utils/pygpt.py
+++ b/py/utils/pygpt.py
@@ -334,6 +334,7 @@
       return self.Unpack(source.read(struct.calcsize(fmt)))
     for f, value in zip(self.FIELDS, struct.unpack(fmt, source)):
       setattr(self, f.name, f.Unpack(value))
+    return None
 
   def Pack(self):
     """Packs values in all fields into a string by struct format."""
@@ -928,6 +929,7 @@
           backup_header.PartitionEntriesStartingLBA)
       WriteData(
           'Backup Header', backup_header.blob, backup_header.CurrentLBA)
+    return None
 
 
 class GPTCommands(object):