Don't call coc#status() if it doesn't exist

plugin/statusline.vim

@@ -64,8 +64,12 @@ function! statusline#generic(group, mode)
   let l:state = '%#StatusLineDark#'
         \.'%{&readonly ? " 🔒" : ""}'
         \.'%{&modifiable ? "" : " ⛔"}'
+  if exists('*coc#status')
     " Display coc.nvim status.
     let l:coc = '%#StatusLineDuskFade#%( %{coc#status()}%)'
+  else
+    let l:coc = ''
+  endif
   " Display filetype if set.
   let l:type = '%#StatusLineDark# %{&filetype} '
   " Display fileencoding if not utf-8 and fileformat if not unix with dusk