Silence warnings about missing dbus-send command not found

Without piping the output to /dev/null, we'd get the following:

  sh: dbus-send: command not found

And if dbus support is only limited to runtime support, we don't
treat it as a fatal error and only notice the user.

Change-Id: Ia7750a074b8d563bf9694fa8fa628b4017e8bb68
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>