Handle FugitiveReal('') correctly

plugin/fugitive.vim

@@ -143,7 +143,7 @@ function! FugitiveReal(...) abort
   let file = a:0 ? a:1 : @%
   if file =~? '^fugitive:' || a:0 > 1
     return call('fugitive#Real', [file] + a:000[1:-1])
-  elseif file =~# '^/\|^\a\+:'
+  elseif file =~# '^/\|^\a\+:\|^$'
     return file
   else
     return fnamemodify(file, ':p' . (file =~# '[\/]$' ? '' : ':s?[\/]$??'))