Fix argument error in streamer

`Repo.exists` can't use `nil` as it is unsafe.
Use parent object instead of activity because currently Announce activity's context is null.

lib/pleroma/web/streamer/streamer.ex

@@ -136,7 +136,7 @@ defmodule Pleroma.Web.Streamer do
          false <- Pleroma.Web.ActivityPub.MRF.subdomain_match?(domain_blocks, item_host),
          false <- Pleroma.Web.ActivityPub.MRF.subdomain_match?(domain_blocks, parent_host),
          true <- thread_containment(item, user),
-         false <- CommonAPI.thread_muted?(user, item) do
+         false <- CommonAPI.thread_muted?(user, parent) do
       false
     else
       _ -> true