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

There was a fix in #1043 which improves the behavior for non saved
filenames. Those are not on disk, but also not empty. This lead to an
issue #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 #1059

CHANGELOG.md

@@ -8,6 +8,7 @@
         - **.PATCH**: Pull Request Title (PR Author) [PR Number](Link to PR)
 -->
 #### 6.4
+- **.6:** NERDTreeFind shows expected message if file doesn't exist e.g. with vim-startify (andys8). [#1081](https://github.com/preservim/nerdtree/pull/1081)
 - **.5**: Ensure events are (or aren't) being ignored correctly. (PhilRunninger) [#1080](https://github.com/preservim/nerdtree/pull/1080)
 - **.4**: Prevent overwriting existing files/dirs on node move. (PhilRunninger) [#1079](https://github.com/preservim/nerdtree/pull/1079)
 - **.3**: Fix regex that finds keyword for minimal menu. (PhilRunninger) [#1075](https://github.com/preservim/nerdtree/pull/1075)

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)