[vim] Ignore 'dir' option if empty

This makes it easier to override FZF command like follows:

    autocmd VimEnter * command! -nargs=? -bang -complete=dir FZF call fzf#run({
          \ 'sink': 'tabe',
          \ 'dir': <q-args>,
          \ 'options': '-m',
          \ 'tmux_height': empty('<bang>') ? '40%' : '' })