Make Element::attributes() less error-prone and simplify call sites

Source/core/dom/AttributeCollection.h

Index: Source/core/dom/AttributeCollection.h
diff --git a/Source/core/dom/AttributeCollection.h b/Source/core/dom/AttributeCollection.h
index 8890e70bbe48e0627bf0b722076d749d73496d3d..860111cae01ec77928853ace81e1dae58bb9471e 100644
--- a/Source/core/dom/AttributeCollection.h
+++ b/Source/core/dom/AttributeCollection.h
@@ -95,6 +95,10 @@ class AttributeCollection : public AttributeCollectionGeneric<const AttributeArr
 public:
     typedef iterator const_iterator;
 
+    AttributeCollection()
+        : AttributeCollectionGeneric<const AttributeArray>(AttributeArray(nullptr, 0))
+    { }
+
     AttributeCollection(const Attribute* array, unsigned size)
         : AttributeCollectionGeneric<const AttributeArray>(AttributeArray(array, size))
     { }