mrf: add no placeholder-text policy, strips pointless "." content from posts with images

lib/pleroma/web/activity_pub/mrf/no_placeholder_text_policy.ex

+# Pleroma: A lightweight social networking server
+# Copyright © 2019 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.MRF.NoPlaceholderTextPolicy do
+  @behaviour Pleroma.Web.ActivityPub.MRF
+
+  @impl true
+  def filter(
+        %{
+          "type" => "Create",
+          "object" => %{"content" => content, "attachment" => _attachment} = child_object
+        } = object
+      )
+      when content in [".", "<p>.</p>"] do
+    child_object =
+      child_object
+      |> Map.put("content", "")
+
+    object =
+      object
+      |> Map.put("object", child_object)
+
+    {:ok, object}
+  end
+
+  @impl true
+  def filter(object), do: {:ok, object}
+end