MythArchive: Fix problem playing created DVD's with no intro

This allows a DVD theme to be created that doesn't have an intro. All of the
default themes have a intro except the auto play theme which doesn't have this
problem so I haven't tested this at all. Fixes #11942.

Signed-off-by: Paul Harrison <pharrison@mythtv.org>
(cherry picked from commit 0a715bf3da19a0ac5e42b3f8ed6133ef71c163fc)