storage: Test DerivedPageDataUpdate behavior around null edits

Why:

- Idf6625d3e737e4a225648b0fd81cc929b095d315 changed
  DerivedPageDataUpdater to enqueue CategoryMembershipChangeJobs for
  "nominal content changes", i.e. if the current change actually changed
  the page content or was a "reconciliation request".
- In DE parlance, null edits are considered to be a form of
  "reconciliation request".
- CategoryMembershipChangeJob is responsible for generating RC entries
  for each category membership change that resulted from an edit, with
  some added heavy lifting to handle multiple edits that occurred in
  quick succession inside a single job. It therefore is not intended to
  run in the context of a reconciliation request, be it a null edit or
  something else, and is better suited to be enqueued from
  ChangeTrackingEventIngress rather than DerivedPageDataUpdater.
- Before we make this change, it would be valuable to add test coverage
  around null edit handling in DerivedPageDataUpdater and to assert on a
  CategoryMembershipChangeJob being enqueued as needed.

What:

- Expand DerivedPageDataUpdater integration tests to cover page creation
  and null edits.
- Add an assertion checking that a CategoryMembershipChangeJob is
  enqueued (or not) as needed.

Bug: T390636
Change-Id: I14944639e00407f59380c8787fd2810c7d24a5c9

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