RB.DraftReviewBannerView instantiates a RB.SplitButtonView, which

wraps the new RB.MenuButtonView. This is a bit more strict than the

older RB.SplitButtonView with DOM elements actually being available on

the page. The problem is that during unit test runs, the HTML backing

RB.DraftReviewBannerView is not available, and this ultimately leads

to assertion failures in the menu button.
This is easily solved by making sure we pass in either a DOM element or

a falsy value, rather than a jQuery object with a length of 0. This

change updates the instantiation to do that.
It also adds a big comment above it to point out this fact, for future

work on the banner. Sooner or later, we're going to want to move this to

a CSS component and change the way it's structured, and this quirk will

be important to note at that time.

All unit tests pass.
Manually tested that the review request and diff viewer pages had a

functioning draft banner and Publish button.