fix(diagnostic): populate data key in DiagnosticChanged autocmd in reset (#20207)

Follow up to #20173.

runtime/lua/vim/diagnostic.lua

@@ -1426,6 +1426,7 @@ function M.reset(namespace, bufnr)
     vim.api.nvim_exec_autocmds('DiagnosticChanged', {
       modeline = false,
       buffer = iter_bufnr,
+      data = { diagnostics = {} },
     })
   end
 end