Revert "Accept empty labels in hostname for compatibility"

RFC1034#section-3.5 doesn't permit empty labels in hostnames. This was
accepted in N for compatibility, but will return null in future releases.

This reverts commit b1e72048.

Bug: 29560247
Change-Id: Ic5cdf193f52fc4e8c09165d47de0fcb9a37e9b78