validate: check ceph_docker_registry_* length

This commit adds a condition to check whether these variables are empty.
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 2b97ac92)

roles/ceph-validate/tasks/main.yml

@@ -123,4 +123,5 @@
     msg: 'ceph_docker_registry_username and/or ceph_docker_registry_password variables need to be set'
   when:
     - ceph_docker_registry_auth | bool
-    - ceph_docker_registry_username is not defined or ceph_docker_registry_password is not defined
+    - (ceph_docker_registry_username is not defined or ceph_docker_registry_password is not defined)
+    - (ceph_docker_registry_username | length == 0 or ceph_docker_registry_password | length == 0)