Summary:
Fixes#606.
If there are no subviews in `UIView`, yoga assumes that `sizeThatFits:` returns `CGSizeZero`. However, according to [the documentation](https://developer.apple.com/documentation/uikit/uiview/1622625-sizethatfits), `UIView` returns current size if there are no subviews.
This diff adds a check - if there are no subviews, `sizeThatFits:` doesn't get called, and CGSizeZero is returned.
Pull Request resolved: https://github.com/facebook/yoga/pull/610
Reviewed By: davidaurelio
Differential Revision: D6807406
Pulled By: priteshrnandgaonkar
fbshipit-source-id: 9189cf14c393f840122bc365d3827881bf03548c
838ef47847