python: Treat None-return of greeting cmd

python: Treat None-return of greeting cmd

In case qemu process dies the "monitor.cmd" returns None which gets
passed to the "__negotiate_capabilities" and leads to unhandled
exception. Let's only check the resp in case it has a value.

Signed-off-by: Lukáš Doktor <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Wainer dos Santos Moschetta <[email protected]>
Message-Id: <20200120071202[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>