Add empty dir icons to docs

README.md

@@ -88,6 +88,8 @@ let g:nvim_tree_icons = {
     \ 'folder': {
     \   'default': "",
     \   'open': "",
+    \    empty = "",
+    \    empty_open = "",
     \   'symlink': "",
     \   }
     \ }

doc/nvim-tree-lua.txt

@@ -107,6 +107,8 @@ You can set icons for:
       \ 'folder': {
       \   'default': "",
       \   'open': "",
+      \    empty = "",
+      \    empty_open = "",
       \   'symlink': "",
       \  }
       \ }