[BUGFIX] Shows error for empty filenames (vim-startify)

There was a fix in preservim#1043 which improves the behavior for non saved
filenames. Those are not on disk, but also not empty. This lead to an
issue preservim#1059 where actual "empty files" like created with vim-startify or
stdin lead to an error. This change fixes this by adapting the order of
the tests. It'll print "no file for the current buffer" for empty files.

Solves preservim#1059

autoload/nerdtree/ui_glue.vim

@@ -284,15 +284,16 @@ endfunction
 " FUNCTION: s:findAndRevealPath(pathStr) {{{1
 function! s:findAndRevealPath(pathStr) abort
     let l:pathStr = !empty(a:pathStr) ? a:pathStr : expand('%:p')
-    if !filereadable(l:pathStr)
-        let l:pathStr = fnamemodify(l:pathStr, ':h')
-    endif
 
     if empty(l:pathStr)
         call nerdtree#echoWarning('no file for the current buffer')
         return
     endif
 
+    if !filereadable(l:pathStr)
+        let l:pathStr = fnamemodify(l:pathStr, ':h')
+    endif
+
     try
         let l:pathStr = g:NERDTreePath.Resolve(l:pathStr)
         let l:pathObj = g:NERDTreePath.New(l:pathStr)