Add empty favicon to prevent `404` error in Firefox dev console

[iandunn]

Firefox automatically requests a favicon, even if a site's HTML doesn't declare one. If the site doesn't have one, that request results in a 404, which creates an error in the dev console. That false error can be distracting to devs who are making changes to a block and watching for errors.

Unfortunately, Firefox doesn't provide a good way to disable that behavior, so the standard workaround is to add a null favicon.

[youknowriad]

Makes sense. We could consider a good default favicon later.