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

Source/core/dom/AttributeCollection.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  * Copyright (C) 2014 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Samsung Electronics. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
@@ skipping 77 matching lines @@
 
 private:
     const Attribute* m_array;
     unsigned m_size;
 };
 
 class AttributeCollection : public AttributeCollectionGeneric<const AttributeArray> {
 public:
     typedef iterator const_iterator;
 
+    AttributeCollection()
+        : AttributeCollectionGeneric<const AttributeArray>(AttributeArray(nullptr, 0))
+    { }
+
     AttributeCollection(const Attribute* array, unsigned size)
         : AttributeCollectionGeneric<const AttributeArray>(AttributeArray(array, size))
     { }
 };
 
 typedef Vector<Attribute, 4> AttributeVector;
 class MutableAttributeCollection : public AttributeCollectionGeneric<AttributeVector, AttributeVector&> {
 public:
     explicit MutableAttributeCollection(AttributeVector& attributes)
         : AttributeCollectionGeneric<AttributeVector, AttributeVector&>(attributes)
@@ skipping 102 matching lines @@
             if (equalPossiblyIgnoringCase(name, it->name().toString(), shouldIgnoreAttributeCase))
                 return index;
         }
     }
     return kNotFound;
 }
 
 } // namespace blink
 
 #endif // AttributeCollection_h