Remove empty container after promoting its children to siblings

This is a proposed fix for issue #460. The cause of the reported crash lies in the order of operations in mu_container_splice_children() and the facts that mu_container_append_siblings() does not allow appending to a NULL container.

The proposed fix changes the order of operations when splicing container's children so that the children are promoted first and then the parent container is removed.

Another way to address this issue would be to make mu_container_append_siblings() gracefully handle appending to a NULL container.