Fixes crash when array is empty (no characters)

@@ -503,10 +503,16 @@ class _Line {
 
   final bool justify;
 
-  double get height => parent._spans
-      .sublist(firstSpan, lastSpan)
+  double get height {
+    final list = parent._spans.sublist(firstSpan, lastSpan);
+    if (list.length > 0) {
+      return list
           .reduce((a, b) => a.height > b.height ? a : b)
           .height;
+    } else {
+      return 0;
+    }
+  }
 
   @override
   String toString() =>