payload: trivial, set to '' for proto

BUG=None
TEST=None

Change-Id: Ica37e09303ded3b44c26fbdc909a4df89129e0cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2537106
Reviewed-by: George Engelbrecht <engeg@google.com>
Tested-by: George Engelbrecht <engeg@google.com>
diff --git a/api/controller/payload.py b/api/controller/payload.py
index 1253406..a32ca36 100644
--- a/api/controller/payload.py
+++ b/api/controller/payload.py
@@ -100,9 +100,9 @@
 
   Args:
     output_proto (PayloadGenerationResult_pb2): The output proto.
-    local_path (str): set output_proto with the local path, or None.
-    remote_uri (str): set output_proto with the remote uri, or None.
+    local_path (str): set output_proto with the local path, or ''.
+    remote_uri (str): set output_proto with the remote uri, or ''.
   """
   output_proto.success = True
-  output_proto.local_path = local_path
-  output_proto.remote_uri = remote_uri
+  output_proto.local_path = local_path or ''
+  output_proto.remote_uri = remote_uri or ''