hass: Omit retain when it is None

Home Assistant refuses to parse entity configuration objects if the
value of the `retain` field is `null`.

src/hass.rs

@@ -34,6 +34,7 @@ pub struct HassConfig {
     pub unique_id: String,
     pub object_id: String,
     pub icon: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub retain: Option<bool>,
 }