Fix String.split when parts and trim is used (#5077)

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

Author: cogle

lib/elixir/lib/string.ex

@@ -350,6 +350,7 @@ defmodule String do
   defp parts_to_index(:infinity),                      do: 0
   defp parts_to_index(n) when is_integer(n) and n > 0, do: n
 
+  defp split_each("", _pattern, true, 1), do: []
   defp split_each(string, _pattern, _trim, 1) when is_binary(string), do: [string]
   defp split_each(string, pattern, trim, count) do
     case do_splitter(string, pattern, trim) do

lib/elixir/test/elixir/string_test.exs

@@ -66,6 +66,9 @@ defmodule StringTest do
     assert String.split("abé", "", trim: true, parts: 2) == ["a", "bé"]
 
     assert String.split("noël", "") == ["n", "o", "ë", "l", ""]
+
+    assert String.split("x-", "-", parts: 2, trim: true) == ["x"]
+    assert String.split("x-x-", "-", parts: 3, trim: true) == ["x", "x"]
   end
 
   test "split with regex" do