Provide hasChildren() to replace isEmptyContent()

Summary:
Fixes T3698. Sometimes views need to render differently depending on whether they contain content or not. The existing approach for this is `isEmptyContent()`, which doesn't work well and is sort of hacky (it implies double-rendering content, which is not always free or side-effect free).

Instead, provide a test for an element without children. This test is powerful enough to catch the easy cases of `null`, etc., and just do the expected thing, but will not catch a View which is reduced upon rendering. Since this is rare and we have no actual need for it today, just accept that as a limitation.

Test Plan:
Viewed Timeline and Feed UI examples. Viewed Feed (feed), Pholio (timelineview), and Differential (old transactionview).

{F53915}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T3698

Differential Revision: https://secure.phabricator.com/D6718