BUG/MEDIUM: resolvers: Accept to create resolution without hostname

BUG/MEDIUM: resolvers: Accept to create resolution without hostname

A regression was introduced by commit 6cf2401ed ("BUG/MEDIUM: resolvers:
Make resolution owns its hostname_dn value"). In fact, it is possible (an
allowed ?!) to create a resolution without hostname (hostname_dn ==
NULL). It only happens on startup for a server relying on a resolver but
defined with an IP address and not a hostname

Because of the patch above, an error is triggered during the configuration
parsing when this happens, while it should be accepted.

This patch must be backported with the commit above.