Fix `device.PushChangedFiles(delete_device_stale=True)` when no device files currently exist

Fix `device.PushChangedFiles(delete_device_stale=True)` when no device files currently exist

It would try to run "rm -f", which fails without any args

BUG=

Committed: https://crrev.com/68a405ad1e13e9874265fc82f02bf55d65cb7ebd
Cr-Commit-Position: refs/heads/master@{#345111}

[agrieve, 2015-08-19 14:09:56]

agrieve@chromium.org changed reviewers:
+ jbudorick@chromium.org

[jbudorick, 2015-08-19 14:14:22]

lgtm

[agrieve, 2015-08-19 18:02:28]

The CQ bit was checked by agrieve@chromium.org

[commit-bot: I haz the power, 2015-08-19 18:02:58]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-19 18:02:59]

This CL has an open dependency (Issue 1291793007 Patch 1). Please resolve the
dependency and try again.

[agrieve, 2015-08-24 17:01:35]

The CQ bit was checked by agrieve@chromium.org

[agrieve, 2015-08-24 17:01:35]

The patchset sent to the CQ was uploaded after l-g-t-m from
jbudorick@chromium.org
Link to the patchset: https://codereview.chromium.org/1297903004/#ps20001
(title: "rebased")

[commit-bot: I haz the power, 2015-08-24 17:02:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1297903004/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1297903004/20001

[commit-bot: I haz the power, 2015-08-24 17:40:16]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-08-24 17:40:44]

Patchset 2 (id:??) landed as
https://crrev.com/68a405ad1e13e9874265fc82f02bf55d65cb7ebd
Cr-Commit-Position: refs/heads/master@{#345111}