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