dirmngr: Allow to pass no filter args to dirmngr_ldap.

* dirmngr/dirmngr_ldap.c (main): Handle no args case.

This is required for example for CRLs. The old code did not require
this because the hos was taken from the URL given has arg.

(cherry picked from commit f6e45671aa26f3e7abb968a876de7bbdb4fca3f1)

• Signed-off-by: Werner Koch <wk@gnupg.org>