Fix return value of String.at/2 with negative position out of bounds

Author: yrashk

lib/elixir/lib/string.ex

@@ -463,7 +463,7 @@ defmodule String do
     real_pos = do_length(next_grapheme(string)) - abs(position)
     case real_pos >= 0 do
       true  -> do_at(next_grapheme(string), real_pos, 0)
-      false -> ""
+      false -> nil
     end
   end
 

lib/elixir/test/elixir/string_test.exs

@@ -162,7 +162,7 @@ defmodule StringTest do
     assert String.at("л", 10) == nil
     assert String.at("elixir", -1) == "r"
     assert String.at("がガちゃ", -2) == "ち"
-    assert String.at("л", -3) == ""
+    assert String.at("л", -3) == nil
     assert String.at("Ā̀stute", 1) == "s"
     assert String.at("elixir",6) == nil
   end