Update common tests (#52)

This pr simplifies decoding logic. The code has worse performance but
better maintainability

.tool-versions

@@ -1,2 +1,2 @@
 erlang 25.0.2
-elixir 1.14.0-otp-25
+elixir 1.14.3-otp-25

lib/ex_rlp.ex

@@ -65,7 +65,7 @@ defmodule ExRLP do
   ## Examples
 
       iex> ExRLP.decode(<<>>)
-      nil
+      ** (ExRLP.DecodeError) invalid rlp encoding
 
       iex> ExRLP.decode(<<0x83, ?d, ?o, ?g>>)
       "dog"

lib/ex_rlp/decode.ex

@@ -3,7 +3,11 @@ defmodule ExRLP.Decode do
 
   alias ExRLP.DecodeItem
   @spec decode(binary(), keyword()) :: ExRLP.t()
-  def decode(item, options \\ []) when is_binary(item) do
+  def decode(item, options \\ [])
+
+  def decode("", _), do: raise(ExRLP.DecodeError)
+
+  def decode(item, options) when is_binary(item) do
     item
     |> unencode(Keyword.get(options, :encoding, :binary))
     |> DecodeItem.decode_item()