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