Have the same behavior for setting a CSS style to null versus empty string in all browsers.

tests/html/cssstyledeclaration_test.dart

Index: tests/html/cssstyledeclaration_test.dart
diff --git a/tests/html/cssstyledeclaration_test.dart b/tests/html/cssstyledeclaration_test.dart
index 38bdbf3b36d9834eb0bc4af78a6a8113690db5de..f506c5c9840e3be515f83ac26b35eb0468db0631 100644
--- a/tests/html/cssstyledeclaration_test.dart
+++ b/tests/html/cssstyledeclaration_test.dart
@@ -55,6 +55,10 @@ main() {
     style.border = "1px solid blue";
     style.border = "";
     expect(style.border, equals(""));
+
+    style.border = "1px solid blue";
+    style.border = null;
+    expect(style.border, equals(""));
   });
 
   test('CSS property getters and setters', () {