upnp: Avoid warning on empty resolve result

Fix warning when browsing a Synology UPnP server which returns
an empty DIDL.

(grilo-test-ui-0.2:7898): Grilo-WARNING **: [upnp] grl-upnp.c:1151: Failed to parse DIDL result: Empty 'DIDL-Lite' node in the DIDL-Lite
XML: <DIDL-Lite xmlns="urn:schemas-upnp-org:metadata-1-0/DIDL-Lite/" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:upnp="urn:schemas-upnp-org:metadata-1-0/upnp/" xmlns:sec="http://www.sec.co.kr/" xmlns:dlna="urn:schemas-dlna-org:metadata-1-0/"></DIDL-Lite>

https://bugzilla.gnome.org/show_bug.cgi?id=704917