docs: xsl: Don't format empty sections in API manual
docs: xsl: Don't format empty sections in API manual

The LXC module has no exported 'Types' but the XSL template which
generates the 'libvirt-libvirt-lxc.html' page would try to format it
anyways. This would result in an empty non-pair version of the '<pre>'
tag to be used in the page, which didn't render well with modern
browsers for some reason. All following sections would become children
of the non-pair <pre>.

Fix the XSL template to not generate empty 'Types' or 'Functions'
sections similarly to how we do with 'Macros'.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

libvirt/libvirt,libvirt/libvirt,jfehlig/libvirt,jfehlig/libvirt,libvirt/libvirt,zippy2/libvirt,zippy2/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,olafhering/libvirt,jfehlig/libvirt,olafhering/libvirt,olafhering/libvirt,zippy2/libvirt,olafhering/libvirt