fix(schema): fix setupTimestamps for browser.umd

[raphael-papazikas]

With the current browser.umd it‘s no longer possible to create schemas in the browser with timestamps: true. The following error is thrown: Cannot read properties of null (reading 'get')

Example to reproduce:

import mongoose from "mongoose";

const TestSchema = new mongoose.Schema(
  { a: String },
  { timestamps: true }
);

[vkarpov15]

Thanks 👍