LSP: Fix nil settings handling in workspace/configuration (#13708)

The `workspace/configuration` handler could fail with the following
error if `config.settings` is nil:

    runtime/lua/vim/lsp/util.lua:1432: attempt to index local 'settings' (a nil value)"

This ensures that `config.settings` is always initialized to an empty
table.

runtime/lua/vim/lsp/handlers.lua

@@ -162,6 +162,7 @@ M['workspace/configuration'] = function(err, _, params, client_id)
   local client = vim.lsp.get_client_by_id(client_id)
   if not client then
     err_message("LSP[id=", client_id, "] client has shut down after sending the message")
+    return
   end
   if err then error(vim.inspect(err)) end
   if not params.items then