fix(types): fix waitForSelector typing to not union null when appropriate

[jharwig]

I found an issue with the types.d.ts file regarding waitForSelector when passing some options, but no state. When no state key is provided, the return type would include null. This is despite the default state value is visible and shouldn't allow null.

I discovered when doing something like below. The example shows passing no options gives the correct return type, but not when options are passed.

Source Code

  const el = await page.waitForSelector(tid('document-viewer'), {
    timeout: docLoadTimeout, // no state passed, should default to visible
  })
  await el.click() // Object is possible 'null'

  const elNoOptions = await page.waitForSelector(tid('document-viewer'))
  await elNoOptions.click()

I found the solution, added some tests, but I also fixed the canBeNull assertion because my new test case wouldn't fail the tests as they were (even though the types clearly included null, but the assertion was missing it.) Looking at some code review comments this might have been the case for a while. If anyone has some insight, I'd love to hear it.

[ghost]

All CLA requirements met.

[JoelEinbinder]

Great patch. Thanks!