Doesn't start adb usb server if name is empty.

Earlier I had tried to disable adb usb by giving an empty string for the
name, but that wasn't enough. Trying to start a second cuttlefish
instance actually tries to use the same name (the empty string) and
fails.

Bug: 78595418
Test: copy launch_cvd to gce, start two instances with adb usb disabled.
Change-Id: I479c0e5ce8284ae7c34d9328ab16ae682bc7c517