Remove TreatNullAs=NullString for HTMLLinkElement

LayoutTests/fast/dom/element-attribute-js-null.html

Index: LayoutTests/fast/dom/element-attribute-js-null.html
diff --git a/LayoutTests/fast/dom/element-attribute-js-null.html b/LayoutTests/fast/dom/element-attribute-js-null.html
index 850191ee6ff319863493a1f42de08753d5f9b443..6808318fe8cc3064879a00fcaca63e3c1651150e 100644
--- a/LayoutTests/fast/dom/element-attribute-js-null.html
+++ b/LayoutTests/fast/dom/element-attribute-js-null.html
@@ -387,14 +387,14 @@
                     type: 'HTMLLinkElement',
                     elementToUse: document.createElement('link'),
                     attributes: [
-                        {name: 'charset', expectedNull: ''},
-                        {name: 'href', expectedNull: ''},
-                        {name: 'hreflang', expectedNull: ''},
-                        {name: 'media', expectedNull: ''},
-                        {name: 'rel', expectedNull: ''},
-                        {name: 'rev', expectedNull: ''},
-                        {name: 'target', expectedNull: ''},
-                        {name: 'type', expectedNull: ''}
+                        {name: 'charset', expectedNull: 'null'},
+                        {name: 'href', expectedNull: 'null', isUrl: true},
+                        {name: 'hreflang', expectedNull: 'null'},
+                        {name: 'media', expectedNull: 'null'},
+                        {name: 'rel', expectedNull: 'null'},
+                        {name: 'rev', expectedNull: 'null'},
+                        {name: 'target', expectedNull: 'null'},
+                        {name: 'type', expectedNull: 'null'}
                     ]
                 },
                 {