serve dlna: fix invalid dc:date for containers

Containers (directories) never had their Date field set, producing
<dc:date>0001-01-01</dc:date> (Go's zero time) in DIDL-Lite metadata.
This invalid date can confuse strict DLNA clients.

Set the dc:date to the directory's modification time, and as a safety
net, omit the dc:date element entirely when the timestamp is zero.

See #9346

cmd/serve/dlna/cds.go

@@ -44,6 +44,7 @@ func (cds *contentDirectoryService) cdsObjectToUpnpavObject(cdsObject object, fi
 	if fileInfo.IsDir() {
 		obj.Class = "object.container.storageFolder"
 		obj.Title = fileInfo.Name()
+		obj.Date = upnpav.Timestamp{Time: fileInfo.ModTime()}
 		childCount, err := cds.countChildren(cdsObject.Path)
 		if err != nil {
 			fs.Debugf(cds, "error counting children of %s: %v", cdsObject.Path, err)

cmd/serve/dlna/upnpav/upnpav.go

@@ -58,7 +58,11 @@ type Timestamp struct {
 	time.Time
 }
 
-// MarshalXML formats the Timestamp per DIDL-Lite spec
+// MarshalXML formats the Timestamp per DIDL-Lite spec.
+// Zero times are omitted to avoid producing invalid dates like "0001-01-01".
 func (t Timestamp) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
+	if t.IsZero() {
+		return nil
+	}
 	return e.EncodeElement(t.Format("2006-01-02"), start)
 }