Make Attr.value/nodeValue/textContent not nullable

Source/core/dom/Attr.idl

Index: Source/core/dom/Attr.idl
diff --git a/Source/core/dom/Attr.idl b/Source/core/dom/Attr.idl
index 7aafbbca683b8b211eb5501bcebfcd2a0824aedc..d1d6d523bebaaa11160f4e15ded2489bfb2aac84 100644
--- a/Source/core/dom/Attr.idl
+++ b/Source/core/dom/Attr.idl
@@ -26,10 +26,9 @@ interface Attr : Node {
     readonly attribute DOMString? prefix;
     readonly attribute DOMString localName;
     readonly attribute DOMString name;
-    // FIXME: value/nodeValue/textContent should not be nullable.
-    [ImplementedAs=valueForBindings, CustomElementCallbacks] attribute DOMString? value;
-    [ImplementedAs=valueForBindings, MeasureAs=AttrNodeValue] attribute DOMString? nodeValue; // legacy alias of .value
-    [ImplementedAs=valueForBindings, MeasureAs=AttrTextContent] attribute DOMString? textContent; // legacy alias of .value
+    [ImplementedAs=valueForBindings, CustomElementCallbacks] attribute DOMString value;
+    [TreatNullAs=EmptyString, ImplementedAs=valueForBindings, MeasureAs=AttrNodeValue] attribute DOMString nodeValue; // legacy alias of .value
+    [TreatNullAs=EmptyString, ImplementedAs=valueForBindings, MeasureAs=AttrTextContent] attribute DOMString textContent; // legacy alias of .value
 
     [MeasureAs=AttributeOwnerElement] readonly attribute Element? ownerElement;