Make unaddressed_message? condsider [] as empty

changelog.d/fix-report-empty-fields.fix

@@ -0,0 +1 @@
+Fix reports being rejected when the activity had an empty CC or TO field (instead of not having them at all)

lib/pleroma/web/activity_pub/utils.ex

@@ -82,7 +82,11 @@ defmodule Pleroma.Web.ActivityPub.Utils do
   def unaddressed_message?(params),
     do:
       [params["to"], params["cc"], params["bto"], params["bcc"]]
-      |> Enum.all?(&is_nil(&1))
+      |> Enum.all?(fn
+        nil -> true
+        [] -> true
+        _ -> false
+      end)
 
   @spec recipient_in_message(User.t(), User.t(), map()) :: boolean()
   def recipient_in_message(%User{ap_id: ap_id} = recipient, %User{} = actor, params),

test/pleroma/web/activity_pub/activity_pub_controller_test.exs

@@ -1205,9 +1205,8 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
           }
         ],
         "actor" => actor.ap_id,
-        "cc" => [
-          reported_user.ap_id
-        ],
+        # CC and TO might either not exist at all, or be empty. We should be able to handle either.
+        # "cc" => [],
         "content" => "test",
         "context" => "context",
         "id" => "http://#{remote_domain}/activities/02be56cf-35e3-46b4-b2c6-47ae08dfee9e",