Fix bug that crashed with "<p> </p>"

lib/outlook/internal_tree/internal_tree_basic.ex

@@ -28,7 +28,7 @@ defmodule Outlook.InternalTree.Basic do
     [ %InternalNode{node | content: case get_sibling_collocation(node.content) do
         :block -> partition_textnodes(node.content)
         :inline -> inline_to_translation_units(node.content)
-        _ -> node  # rare case of only whitespace textnode(s) - does this ever happen?
+        _ -> [ node ]
       end
     } | partition_textnodes(rest) ]
   end