[dom] Normalize setting a node value to null

When set a node value to `null`, then a node value returns `"null"` as string on the legacy IE (IE <= 9). A node value returns an empty string on the modern browsers (Chrome, Firefox, IE >= 10). This change is to normalize the results of setting a node value to `null` on any browsers. Fix #1603.

Tested with IE6, IE7, IE8, IE9, IE10, IE11, Chrome, Firefox, Safari 7(OS X), Safari 7(iOS).