Fix crash truncating a string of a node with a zero size

The crash happens in the placeholderImage of ASTextNode. The node is not visible in the time it try to get the `placeholderImage` and so the `visibleRange` has count 0 and a crash happens while accessing the first object of an empty array.

AsyncDisplayKit/TextKit/ASTextKitRenderer.mm

@@ -262,3 +262,17 @@ static NSCharacterSet *_defaultAvoidTruncationCharacterSet()
 }
 
 @end
+
+@implementation ASTextKitRenderer (ASTextKitRendererConvenience)
+
+- (NSRange)firstVisibleRange
+{
+  std::vector<NSRange> visibleRanges = self.visibleRanges;
+  if (visibleRanges.size() > 0) {
+    return visibleRanges[0];
+  }
+  
+  return NSMakeRange(NSNotFound, 0);
+}
+
+@end