Completely omit id for anonymous objects

Current AP spec demands anonymous objects to have an id value,
but explicitly set it to JSON null. Howeveras it turns out this is
incompatible with JSON-LD requiring `@id` to be a string and thus AP
spec is incompatible iwth the Ativity Streams spec it is based on.
This is an issue for (the few) AP implementers actually performing
JSON-LD processing, like IceShrimp.NET.
This was uncovered by IceShrimp.NET’s zotan due to our adoption of
anonymous objects for emoj in f101886709.

The issues is being discussed by W3C, and will most likely be resolved
via an errata redefining anonymous objects to completely omit the id
field just like transient objects already do. See:
https://github.com/w3c/activitypub/issues/476

Fixes: #848

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -950,8 +950,7 @@ defp build_emoji_tag({name, url}) do
       "icon" => %{"url" => "#{URI.encode(url)}", "type" => "Image"},
       "name" => ":" <> name <> ":",
       "type" => "Emoji",
-      "updated" => "1970-01-01T00:00:00Z",
-      "id" => nil
+      "updated" => "1970-01-01T00:00:00Z"
     }
   end
 

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

@@ -698,7 +698,6 @@ test "take_emoji_tags/1" do
     assert Transmogrifier.take_emoji_tags(user) == [
              %{
                "icon" => %{"type" => "Image", "url" => "https://example.org/firefox.png"},
-               "id" => nil,
                "name" => ":firefox:",
                "type" => "Emoji",
                "updated" => "1970-01-01T00:00:00Z"

test/pleroma/web/activity_pub/views/user_view_test.exs

@@ -47,7 +47,6 @@ test "Renders with emoji tags" do
              "tag" => [
                %{
                  "icon" => %{"type" => "Image", "url" => "/test"},
-                 "id" => nil,
                  "name" => ":bib:",
                  "type" => "Emoji",
                  "updated" => "1970-01-01T00:00:00Z"