Relax dns search to accept empty domain

In that case /etc/resolv.conf will be generated with no search
option. Usage: --dns-search=.

Docker-DCO-1.1-Signed-off-by: Fabio Falci <fabiofalci@gmail.com> (github: fabiofalci)
Merge pull request #6720 from fabiofalci/relax_dns_search

Relax dns search to accept empty domain