[MPRIS Data Engine] Don't crash if "Metadata" is a map but of wrong type

QDBusArgument MapType fits all kinds of maps but when we try to stream a map of type
other than a{sv} (which the MRPIS spec requires) into a QVariantMap we would crash.
Explicitly check signature to avoid this.

BUG: 374531
FIXED-IN: 5.8.6