Treat empty maps as nils

lib/pleroma/ecto_type/activity_pub/object_validators/map_of_string.ex

@@ -22,7 +22,11 @@ def cast(object) when is_map(object) do
           acc
       end)
 
-    {:ok, data}
+    if data == %{} do
+      {:ok, nil}
+    else
+      {:ok, data}
+    end
   end
 
   def cast(_), do: :error

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -345,7 +345,8 @@ def fix_tag(object), do: object
   def fix_content_map(%{"content" => content} = object) when not_empty_string(content), do: object
 
   # content map usually only has one language so this will do for now.
-  def fix_content_map(%{"contentMap" => content_map} = object) do
+  def fix_content_map(%{"contentMap" => content_map} = object)
+      when is_map(content_map) and content_map != %{} do
     content_groups = Map.to_list(content_map)
     {_, content} = Enum.at(content_groups, 0)
 

test/pleroma/web/activity_pub/object_validators/article_note_page_validator_test.exs

@@ -62,6 +62,21 @@ test "Note with contentMap and summaryMap", %{note: note} do
                }
              } = ArticleNotePageValidator.cast_and_validate(note)
     end
+
+    test "Note with empty *Map", %{note: note} do
+      note =
+        note
+        |> Map.put("summaryMap", %{"und" => "mew"})
+        |> Map.put("contentMap", %{})
+
+      assert %{
+               valid?: true,
+               changes: changes
+             } = ArticleNotePageValidator.cast_and_validate(note)
+
+      assert Map.has_key?(changes, :summaryMap)
+      refute Map.has_key?(changes, :contentMap)
+    end
   end
 
   describe "Note with history" do