B Providers/ActivityPub: Ensure that nothing explodes on unexpected input.

lib/pleroma/web/metadata/providers/activity_pub.ex

@@ -16,4 +16,7 @@ def build_tags(%{object: %{data: %{"id" => object_id}}}) do
   def build_tags(%{user: user}) do
     [{:link, [rel: "alternate", type: "application/activity+json", href: user.ap_id], []}]
   end
+
+  @impl Provider
+  def build_tags(_), do: []
 end

test/pleroma/web/metadata/providers/activity_pub_test.exs

@@ -31,4 +31,10 @@ test "it renders a link for a post" do
               [rel: "alternate", type: "application/activity+json", href: object.data["id"]], []}
            ] == result
   end
+
+  test "it returns an empty array for anything else" do
+    result = ActivityPub.build_tags(%{})
+
+    assert result == []
+  end
 end