R1:fe66d52a22a5

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