lnwire: add method ParseAndExtractExtraData

Similar to `ParseAndExtractCustomRecords`, we now add this helper method
to make sure the extra data is parsed correctly.

lnwire/test_message.go

@@ -881,10 +881,7 @@ func (dp *DynPropose) RandTestMessage(t *rapid.T) Message {
 		}
 	}
 
-	extraData := RandExtraOpaqueData(t, ignoreRecords)
-	if len(extraData) > 0 {
-		msg.ExtraData = extraData
-	}
+	msg.ExtraData = RandExtraOpaqueData(t, ignoreRecords)
 
 	return msg
 }