LibWeb+LibJS: Add [LegacyNullToEmptyString] attribute

If specified, to_string() returns an empty string instead of "null" for
null values.

Libraries/LibJS/Runtime/Value.cpp

@@ -143,13 +143,13 @@ PrimitiveString* Value::to_primitive_string(GlobalObject& global_object)
     return js_string(global_object.heap(), string);
 }
 
-String Value::to_string(GlobalObject& global_object) const
+String Value::to_string(GlobalObject& global_object, bool legacy_null_to_empty_string) const
 {
     switch (m_type) {
     case Type::Undefined:
         return "undefined";
     case Type::Null:
-        return "null";
+        return !legacy_null_to_empty_string ? "null" : String::empty();
     case Type::Boolean:
         return m_value.as_bool ? "true" : "false";
     case Type::Number:

Libraries/LibJS/Runtime/Value.h

@@ -241,7 +241,7 @@ public:
     i32 as_i32() const;
     size_t as_size_t() const;
 
-    String to_string(GlobalObject&) const;
+    String to_string(GlobalObject&, bool legacy_null_to_empty_string = false) const;
     PrimitiveString* to_primitive_string(GlobalObject&);
     Value to_primitive(PreferredType preferred_type = PreferredType::Default) const;
     Object* to_object(GlobalObject&) const;

Libraries/LibWeb/CodeGenerators/WrapperGenerator.cpp

@@ -594,11 +594,12 @@ static @fully_qualified_name@* impl_from(JS::VM& vm, JS::GlobalObject& global_ob
 )~~~");
     }
 
-    auto generate_to_cpp = [&](auto& parameter, auto& js_name, const auto& js_suffix, auto cpp_name, bool return_void = false) {
+    auto generate_to_cpp = [&](auto& parameter, auto& js_name, const auto& js_suffix, auto cpp_name, bool return_void = false, bool legacy_null_to_empty_string = false) {
         auto scoped_generator = generator.fork();
         scoped_generator.set("cpp_name", cpp_name);
         scoped_generator.set("js_name", js_name);
         scoped_generator.set("js_suffix", js_suffix);
+        scoped_generator.set("legacy_null_to_empty_string", legacy_null_to_empty_string ? "true" : "false");
         scoped_generator.set("parameter.type.name", parameter.type.name);
 
         if (return_void)
@@ -608,7 +609,7 @@ static @fully_qualified_name@* impl_from(JS::VM& vm, JS::GlobalObject& global_ob
 
         if (parameter.type.name == "DOMString") {
             scoped_generator.append(R"~~~(
-    auto @cpp_name@ = @js_name@@js_suffix@.to_string(global_object);
+    auto @cpp_name@ = @js_name@@js_suffix@.to_string(global_object, @legacy_null_to_empty_string@);
     if (vm.exception())
         @return_statement@
 )~~~");
@@ -785,7 +786,7 @@ JS_DEFINE_NATIVE_SETTER(@wrapper_class@::@attribute.setter_callback@)
         return;
 )~~~");
 
-            generate_to_cpp(attribute, "value", "", "cpp_value", true);
+            generate_to_cpp(attribute, "value", "", "cpp_value", true, attribute.extended_attributes.contains("LegacyNullToEmptyString"));
 
             if (attribute.extended_attributes.contains("Reflect")) {
                 if (attribute.type.name != "boolean") {