user: remote actors can have no nickname if they are virtual services

lib/pleroma/user.ex

@@ -77,7 +77,7 @@ defmodule Pleroma.User do
     changes =
       %User{}
       |> cast(params, [:bio, :name, :ap_id, :nickname, :info, :avatar])
-      |> validate_required([:name, :ap_id, :nickname])
+      |> validate_required([:name, :ap_id])
       |> unique_constraint(:nickname)
       |> validate_format(:nickname, @email_regex)
       |> validate_length(:bio, max: 5000)

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -554,12 +554,19 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
         "locked" => locked
       },
       avatar: avatar,
-      nickname: "#{data["preferredUsername"]}@#{URI.parse(data["id"]).host}",
       name: data["name"],
       follower_address: data["followers"],
       bio: data["summary"]
     }
 
+    # nickname can be nil because of virtual actors
+    user_data =
+      if data["preferredUsername"] do
+        Map.put(user_data, :nickname, "#{data["preferredUsername"]}@#{URI.parse(data["id"]).host}")
+      else
+        Map.put(user_data, :nickname, nil)
+      end
+
     {:ok, user_data}
   end