[vim] fzf#run should ignore empty 'dir' argument

Fix #2343

README-VIM.md

@@ -362,7 +362,7 @@ Our `:LS` command will be much more useful if we can pass a directory argument
 to it, so that something like `:LS /tmp` is possible.
 
 ```vim
-command! -bang -complete=dir -nargs=* LS
+command! -bang -complete=dir -nargs=? LS
     \ call fzf#run(fzf#wrap({'source': 'ls', 'dir': <q-args>}, <bang>0))
 ```
 
@@ -372,7 +372,7 @@ a unique name to our command and pass it as the first argument to `fzf#wrap`.
 ```vim
 " The query history for this command will be stored as 'ls' inside g:fzf_history_dir.
 " The name is ignored if g:fzf_history_dir is not defined.
-command! -bang -complete=dir -nargs=* LS
+command! -bang -complete=dir -nargs=? LS
     \ call fzf#run(fzf#wrap('ls', {'source': 'ls', 'dir': <q-args>}, <bang>0))
 ```
 

plugin/fzf.vim

@@ -424,10 +424,10 @@ try
     throw v:exception
   endtry
 
-  if !has_key(dict, 'dir')
+  if !s:present(dict, 'dir')
     let dict.dir = s:fzf_getcwd()
   endif
-  if has('win32unix') && has_key(dict, 'dir')
+  if has('win32unix') && s:present(dict, 'dir')
     let dict.dir = fnamemodify(dict.dir, ':p')
   endif