fix: allow html_to_text to receive nil, e.g. for empty event descriptions

Fixes potsda.mn/mobilizon#34

lib/service/formatter/html.ex

@@ -42,6 +42,7 @@ defmodule Mobilizon.Service.Formatter.HTML do
   def strip_tags_and_insert_spaces(html), do: html
 
   @spec html_to_text(String.t()) :: String.t()
+  def html_to_text(nil), do: nil
   def html_to_text(html) do
     html
     |> String.replace(~r/<li>/, "\\g{1}- ", global: true)