Only reload the colorscheme if one is defined.

E.g. work with the default one which doesn't define `g:colors_name`.

plugin/goyo.vim

@@ -259,7 +259,9 @@ function! s:goyo_off()
   for [k, v] in items(goyo_revert)
     execute printf("let &%s = %s", k, string(v))
   endfor
+  if exists('g:colors_name')
     execute 'colo '. g:colors_name
+  endif
 
   if goyo_disabled_gitgutter
     silent! GitGutterEnable