If rev_content_model is null, use page_content_model.

If rev_content_model is null, use page_content_model.

We currently leave rev_content_model = null  if it'S the default, to save space.
When loading a revision, we so far fell back to the per-namespace default content
model if rev_content_model was null, even if page_content_model was null.

This patch changes the fallback from rev_contentModel -> namespace-default-model
to rev_content_model -> page_content_model -> namespace-default-model.

This will prevent errors triggered when chaing a namespace'sdefault content model:
so far, revisions with the old content model would fail to deserialize becaue,
because they were being interpreted according to the changed namespace defrault,
instead of the correct model in page_content_model.

Bug: T128466
Change-Id: I75c60eb129428b0b433480443ab9153cc58cda8f