listrunner: Don't pass arguments if there are none

If no arguments were specified the “exec_args” variable was “None”,
leading to the command being run as “… ./… None”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>