fix for copying files

when copying a file outside the current tree root, {} is returned by
TreeFileNode.copy() so try to reposition the cursor

nerdtree_plugin/fs_menu.vim

@@ -179,8 +179,10 @@ function! NERDTreeCopyNode()
         if confirmed
             try
                 let newNode = currentNode.copy(newNodePath)
+                if !empty(newNode)
                     call NERDTreeRender()
                     call newNode.putCursorHere(0, 0)
+                endif
             catch /^NERDTree/
                 call s:echoWarning("Could not copy node")
             endtry