In the case of multi-channels transcoding, a context with child
sesseion can be parent for others, so we need to check if the
msdkcontext has any child session in the list to avoid session
leaks. Otherwise, we will see the failure of closing a parent
session because one of its child's child session not released.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6259>
41c52f5739