ensure old_content to be binary only when adb_keys exist

This is a hot fix for https://console.cloud.google.com/errors/CJn03NGdqK2lQQ?time=PT1H&refresh=auto&project=chromium-swarm
It can't retrieve adb_keys from devices when they aren't connected.

Bug: 1198820
Change-Id: I9e22ddb363ce53a1823e440cae8fe2e5cf25a95a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/python-adb/+/2931837
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
1 file changed