R1:8ac2da9850b0

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…