Otherwise it'd get stringified to "None" and make resulting DEPS invalid.

In theory we could properly serialize them to None,
but it's simpler to just skip them, since they do not affect resulting checkout.

Bug: 570091
Change-Id: Ica301e2d7e40ba4ed04527355d8f75a43456391c
Reviewed-on: https://chromium-review.googlesource.com/610150Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>