Generate an error when parsing a feed with no data

lib/feed_parser.ex

@@ -23,7 +23,13 @@ defmodule FeedParser do
   """
   @spec parse(data :: String.t(), content_type :: String.t(), parsers :: [module()]) ::
           {:ok, feed :: FeedParser.Feed.t()} | {:error, reason :: String.t()}
-  def parse(data, content_type, parsers \\ @default_parsers) when is_binary(data) do
+  def parse(data, content_type, parsers \\ @default_parsers)
+
+  def parse(nil, _, _) do
+    {:error, "no data"}
+  end
+
+  def parse(data, content_type, parsers) when is_binary(data) do
     parsers
     |> Enum.reduce_while(false, fn parser, acc ->
       case parser.accepts(data, content_type) do