call nvim_command(wa) using pcall() to prevent unnamed buffer error from preventing session from saving

lua/persisted/init.lua

@@ -136,7 +136,7 @@ function M.save(opts)
     if type(config.save_pre) == "function" then
       config.save_pre()
     end
-    vim.api.nvim_command("wa")
+    pcall(vim.api.nvim_command, "wa")
     vim.api.nvim_command("mksession! " .. e(opts.session or vim.g.persisting_session or M.current()))
   end)
 end