Views blocks' `items_per_page` setting can no longer be `none`

Views blocks always have an items_per_page setting -- that is, the number of items from the view that should be visible in the block.

Until #3520946: Make View blocks (block.settings.view_block:*) fully validatable, that setting could either be a number, or none to defer to whatever the view itself has configured. Now, none is no longer supported; to defer to the view, items_per_page should be null.