fix(subsonic): always include mandatory title field in Child responses

Removed `omitempty` from the `Title` struct tag in the `Child` response
type. The Subsonic/OpenSubsonic API spec requires `title` to be a
mandatory field, but songs with empty titles caused the field to be
omitted entirely, crashing clients like Symfonium during sync.

Ref: https://support.symfonium.app/t/app-gets-stuck-on-syncing-large-database/13004/8

server/subsonic/responses/responses.go

@@ -135,7 +135,7 @@ type Child struct {
 	Id                    string     `xml:"id,attr"                                 json:"id"`
 	Parent                string     `xml:"parent,attr,omitempty"                   json:"parent,omitempty"`
 	IsDir                 bool       `xml:"isDir,attr"                              json:"isDir"`
-	Title                 string     `xml:"title,attr,omitempty"                    json:"title,omitempty"`
+	Title                 string     `xml:"title,attr"                              json:"title"`
 	Name                  string     `xml:"name,attr,omitempty"                     json:"name,omitempty"`
 	Album                 string     `xml:"album,attr,omitempty"                    json:"album,omitempty"`
 	Artist                string     `xml:"artist,attr,omitempty"                   json:"artist,omitempty"`