Support objects with a nil contentMap (firefish)

Closes: https://git.pleroma.social/pleroma/pleroma/-/issues/3231

changelog.d/nil-content-map.fix

@@ -0,0 +1 @@
+Support objects with a null contentMap (firefish)

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -339,6 +339,10 @@ def fix_tag(%{"tag" => %{} = tag} = object) do
 
   def fix_tag(object), do: object
 
+  def fix_content_map(%{"contentMap" => nil} = object) do
+    Map.drop(object, ["contentMap"])
+  end
+
   # content map usually only has one language so this will do for now.
   def fix_content_map(%{"contentMap" => content_map} = object) do
     content_groups = Map.to_list(content_map)

test/pleroma/web/activity_pub/transmogrifier/note_handling_test.exs

@@ -221,6 +221,19 @@ test "it works for incoming notices with contentMap" do
                "<p><span class=\"h-card\"><a href=\"http://localtesting.pleroma.lol/users/lain\" class=\"u-url mention\">@<span>lain</span></a></span></p>"
     end
 
+    test "it works for incoming notices with a nil contentMap (firefish)" do
+      data =
+        File.read!("test/fixtures/mastodon-post-activity-contentmap.json")
+        |> Jason.decode!()
+        |> Map.put("contentMap", nil)
+
+      {:ok, %Activity{data: data, local: false}} = Transmogrifier.handle_incoming(data)
+      object = Object.normalize(data["object"], fetch: false)
+
+      assert object.data["content"] ==
+               "<p><span class=\"h-card\"><a href=\"http://localtesting.pleroma.lol/users/lain\" class=\"u-url mention\">@<span>lain</span></a></span></p>"
+    end
+
     test "it works for incoming notices with to/cc not being an array (kroeg)" do
       data = File.read!("test/fixtures/kroeg-post-activity.json") |> Jason.decode!()