Fixes #606 (#610)

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

YogaKit/Tests/YogaKitTests.m

@@ -127,6 +127,15 @@
   XCTAssertEqual(longTextLabelSize.width + textBadgeView.yoga.intrinsicSize.width, containerSize.width);
 }
 
+- (void)testSizeThatFitsEmptyView {
+  UIView* view = [[UIView alloc] initWithFrame:CGRectMake(10, 10, 200, 200)];
+  view.yoga.isEnabled = YES;
+
+  const CGSize viewSize = view.yoga.intrinsicSize;
+  XCTAssertEqual(viewSize.height, 0);
+  XCTAssertEqual(viewSize.width, 0);
+}
+
 - (void)testPreservingOrigin
 {
   UIView *container = [[UIView alloc] initWithFrame:CGRectMake(0,0,50,75)];