Use primary tag name as marker title where title is empty (#2994)

* Fix display of marker popovers
* Use primary tag as title where marker title empty

graphql/documents/data/scene-slim.graphql

@@ -31,6 +31,10 @@ fragment SlimSceneData on Scene {
     id
     title
     seconds
+    primary_tag {
+      id
+      name
+    }
   }
 
   galleries {