Don't copy null attributes passed to JX.$N()

Summary:
Fixes T8919. In Safari, `node.href = null;` has no effect, but in Chrome it is like `node.href = "null";`.

Instead, just use semantics similar to `phutil_tag()`: don't assign attributes with `null` values.

Test Plan:
No more `/null` href in Chrome in Owners typehaead.

Typeahead still works in Chrome/Safari.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8919

Differential Revision: https://secure.phabricator.com/D14021

webroot/rsrc/externals/javelin/lib/DOM.js

@@ -310,7 +310,13 @@ JX.$N = function(tag, attr, content) {
     }
   }
 
-  JX.copy(node, attr);
+  for (var k in attr) {
+    if (attr[k] === null) {
+      continue;
+    }
+    node[k] = attr[k];
+  }
+
   if (content) {
     JX.DOM.setContent(node, content);
   }