fix(ngOptions): don't add comment nodes as empty options

When the "empty option" element contains a transclusion directive, the result of the compilation always includes a comment node. Since we are adding / removing the "selected" attribute on the empty option, we need to make sure it's an actual element.

To solve this, we take advantage of the fact the each option element has an option directive that tries to register the option with the selectController. With ngOptions, this registerOption function is normally noop'd since it's not possible to add dynamic options. Now if the result of the empty option compilation is a comment, we re-define the function so that it catches empty options when they are actually linked / rendered.

Closes #15454
Closes #15456