Try to make sure that clusters center is never null. (#1942)

And/or that at least clusters without a center are never returned.

We don't know exactly in which cases this crash happens. So this commit approaches it in two different ways:

* For a given feed, only return clusters that have a center, by adding a `INNER JOIN project_medias` clause to the feed clusters query.
* For the `ClusterType` GraphQL endpoint, fallback to the cluster model `center` method, which already falls back to the first item of the cluster if the center is not present anymore.

Fixes CV2-4850.

app/graph/types/cluster_type.rb

@@ -18,12 +18,6 @@ def fact_checks_count
 
   field :center, ProjectMediaType, null: true
 
-  def center
-    RecordLoader
-      .for(ProjectMedia)
-      .load(object.project_media_id)
-  end
-
   field :first_item_at, GraphQL::Types::Int, null: true
 
   def first_item_at

app/models/feed.rb

@@ -140,7 +140,7 @@ def clusters_count(args = {})
   def filtered_clusters(args = {})
     team_ids = args[:team_ids]
     channels = args[:channels]
-    query = self.clusters
+    query = self.clusters.joins(:project_media)
 
     # Filter by workspace
     query = query.where.not("ARRAY[?] && team_ids", self.team_ids - team_ids.to_a.map(&:to_i)) if !team_ids.blank? && team_ids != self.team_ids