Nodeinfo: remove null features; relay feature.

lib/pleroma/web/nodeinfo/nodeinfo_controller.ex

@@ -71,7 +71,8 @@ def nodeinfo(conn, %{"version" => "2.0"}) do
         %{}
       end
 
-    features = [
+    features =
+      [
         "pleroma_api",
         "mastodon_api",
         "mastodon_api_streaming",
@@ -86,8 +87,12 @@ def nodeinfo(conn, %{"version" => "2.0"}) do
         end,
         if Keyword.get(suggestions, :enabled) do
           "suggestions"
+        end,
+        if Keyword.get(instance, :allow_relay) do
+          "relay"
         end
       ]
+      |> Enum.filter(& &1)
 
     response = %{
       version: "2.0",