Make the default `payload_filename` be None.

This is a speculative fix to address ongoing failures in the lab.
It appears that the standard provisioning flow is passing a
"payload_filename" parameter that's an empty string instead of None.
That's causing the AU EndToEnd test flow to trigger, causing failures.

TEST=None
BUG=chromium:744959

Change-Id: I142aa0c24b1a2305c51fae0a78762e0c92546792
Reviewed-on: https://chromium-review.googlesource.com/575429
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Richard Barnette <jrbarnette@google.com>

cros_update.py

diff --git a/cros_update.py b/cros_update.py
index 528e51f..3bd0aa5 100644
--- a/cros_update.py
+++ b/cros_update.py
@@ -94,7 +94,7 @@
                              help=('force stateful update with the same '
                                    'version of previous rootfs partition'))
     self.parser.add_argument('--payload_filename', action='store', type=str,
-                             dest='payload_filename', default='',
+                             dest='payload_filename', default=None,
                              help='A custom payload filename')
     self.parser.add_argument('--clobber_stateful', action='store_true',
                              dest='clobber_stateful', default=False,