Ensure `remove_tag` returns a valid html_tree

If the entire input is stripped out, this used to return `nil` which
caused downstream parsing to fail. Instead, return `[]` which is the
Floki representation of an empty tree.

Fixes #36

lib/readability/helper.ex

@@ -64,7 +64,7 @@ defmodule Readability.Helper do
   def remove_tag([h | t], fun) do
     node = remove_tag(h, fun)
 
-    if is_nil(node) do
+    if node == [] do
       remove_tag(t, fun)
     else
       [node | remove_tag(t, fun)]
@@ -73,7 +73,7 @@ defmodule Readability.Helper do
 
   def remove_tag({tag, attrs, inner_tree} = html_tree, fun) do
     if fun.(html_tree) do
-      nil
+      []
     else
       {tag, attrs, remove_tag(inner_tree, fun)}
     end

test/readability/helper_test.exs

@@ -45,6 +45,19 @@ defmodule Readability.HelperTest do
     assert result == expected
   end
 
+  test "remove all tags", %{html_tree: html_tree} do
+    expected = "" |> parse
+
+    result =
+      html_tree
+      |> Helper.remove_tag(fn {tag, _, _} ->
+        tag == "html"
+      end)
+
+    assert result == expected
+  end
+
+
   test "inner text length", %{html_tree: html_tree} do
     result = html_tree |> Helper.text_length()
     assert result == 5