add nil clause for Formatter.get_emoji/1 to return an empty result

lib/pleroma/formatter.ex

@@ -157,6 +157,8 @@ defmodule Pleroma.Formatter do
     end)
   end
 
+  def get_emoji(nil), do: []
+
   def get_emoji(text) do
     Enum.filter(@emoji, fn {emoji, _} -> String.contains?(text, ":#{emoji}:") end)
   end