Update Process.info spec to reflect the possible nil return value (#7977)

The @SPEC for it did not reflect that possible return value - which causes dialyzer to throw errors if you check for that condition. Now the spec allows for a return value of nil.

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: boydm

lib/elixir/lib/process.ex

@@ -636,7 +636,7 @@ defmodule Process do
 
   See `:erlang.process_info/1` for more info.
   """
-  @spec info(pid) :: keyword
+  @spec info(pid) :: keyword | nil
   def info(pid) do
     nillify(:erlang.process_info(pid))
   end