Avoid querying for NSAttributedString attributes on empty string

[rahul-malik]

Resolves crash with stack trace:

Fatal Exception: NSRangeException
NSRLEArray objectAtIndex:effectiveRange:: Out of bounds

0  CoreFoundation                 0x1eab6298c __exceptionPreprocess
1  libobjc.A.dylib                0x1e9d3b9f8 objc_exception_throw
2  CoreFoundation                 0x1eaa6cbc0 -[NSCache init]
3  Foundation                     0x1eb4e4a7c -[NSRLEArray objectAtIndex:effectiveRange:]
4  Foundation                     0x1eb4e4c34 -[NSConcreteAttributedString attribute:atIndex:effectiveRange:]
5  PinterestEnterprise            0x10419ecdc -[ASTextKitContext initWithAttributedString:tintColor:lineBreakMode:maximumNumberOfLines:exclusionPaths:constrainedSize:] + 68 (ASTextKitContext.mm:68)
6  PinterestEnterprise            0x1041a3c48 -[ASTextKitRenderer initWithTextKitAttributes:constrainedSize:] + 63 (ASTextKitRenderer.mm:63)
7  PinterestEnterprise            0x104107d08 rendererForAttributes(ASTextKitAttributes, CGSize) + 126 (ASTextNode.mm:126)
8  PinterestEnterprise            0x104108bb8 -[ASTextNode _locked_rendererWithBounds:] + 367 (ASTextNode.mm:367)
9  PinterestEnterprise            0x104108fa8 -[ASTextNode calculateSizeThatFits:] + 426 (ASTextNode.mm:426)
10 PinterestEnterprise            0x1040d4828 -[ASDisplayNode(ASLayoutSpec) calculateLayoutLayoutSpec:] + 43 (ASDisplayNode+LayoutSpec.mm:43)
11 PinterestEnterprise            0x1040d7f58 -[ASDisplayNode calculateLayoutThatFits:] + 1056 (ASDisplayNode.mm:1056)
12 PinterestEnterprise            0x1040d7ebc -[ASDisplayNode calculateLayoutThatFits:restrictedToSize:relativeToParentSize:] + 1048 (ASDisplayNode.mm:1048)
13 PinterestEnterprise            0x1040d07cc -[ASDisplayNode(ASLayoutElement) layoutThatFits:parentSize:] + 109 (ASDisplayNode+Layout.mm:109)
14 PinterestEnterprise            0x10416dd68 crossChildLayout(ASStackLayoutSpecChild const&, ASStackLayoutSpecStyle const&, double, double, double, double, CGSize) + 65 (ASStackUnpositionedLayout.mm:65)
15 PinterestEnterprise            0x10416da98 invocation function for block in layoutItemsAlongUnconstrainedStackDimension(std::__1::vector<ASStackLayoutSpecItem, std::__1::allocator<ASStackLayoutSpecItem> >&, ASStackLayoutSpecStyle const&, bool, ASSizeRange const&, CGSize, bool) + 689 (ASStackUnpositionedLayout.mm:689)
16 PinterestEnterprise            0x10416d900 dispatchApplyIfNeeded(unsigned long, bool, void (unsigned long) block_pointer) + 83 (ASStackUnpositionedLayout.mm:83)
17 PinterestEnterprise            0x10416cbd8 ASStackUnpositionedLayout::compute(std::__1::vector<ASStackLayoutSpecChild, std::__1::allocator<ASStackLayoutSpecChild> > const&, ASStackLayoutSpecStyle const&, ASSizeRange const&, bool) + 636 (ASStackUnpositionedLayout.mm:636)
18 PinterestEnterprise            0x104154e8c -[ASStackLayoutSpec calculateLayoutThatFits:] + 149 (ASStackLayoutSpec.mm:149)
19 PinterestEnterprise            0x104151c68 -[ASLayoutSpec calculateLayoutThatFits:restrictedToSize:relativeToParentSize:] + 80 (ASLayoutSpec.mm:80)
20 PinterestEnterprise            0x104151af8 -[ASLayoutSpec layoutThatFits:parentSize:] + 80 (ASLayoutSpec.mm:80)
21 PinterestEnterprise            0x10414b9e8 -[ASInsetLayoutSpec calculateLayoutThatFits:restrictedToSize:relativeToParentSize:] + 102 (ASInsetLayoutSpec.mm:102)
22 PinterestEnterprise            0x104151af8 -[ASLayoutSpec layoutThatFits:parentSize:] + 80 (ASLayoutSpec.mm:80)
23 PinterestEnterprise            0x1040d49a8 -[ASDisplayNode(ASLayoutSpec) calculateLayoutLayoutSpec:] + 93 (ASDisplayNode+LayoutSpec.mm:93)
24 PinterestEnterprise            0x1040d7f58 -[ASDisplayNode calculateLayoutThatFits:] + 1056 (ASDisplayNode.mm:1056)
25 PinterestEnterprise            0x1040d7ebc -[ASDisplayNode calculateLayoutThatFits:restrictedToSize:relativeToParentSize:] + 1048 (ASDisplayNode.mm:1048)

[nguyenhuy]

LGTM. Thanks for adding new tests!