fix a crash when closing a picker with no matches

#60
it was reverted in the original repo with fea272d
but i still think this should be deleted and that fixes the close issue
open a picker, type text until there are no matches, close the picker

Author: emmanueltouzery

lua/nvim-devdocs/pickers.lua

@@ -48,7 +48,7 @@ local metadata_previewer = previewers.new_buffer_previewer({
 
 local doc_previewer = previewers.new_buffer_previewer({
   title = "Preview",
-  keep_last_buf = true,
+  -- keep_last_buf = true,
   define_preview = function(self, entry)
     local bufnr = self.state.bufnr