[discover_latest_image] Don't set single-image facts to null

Setting the single-image facts to `null` broke some `when` conditions
that expected the facts to be completely unset.

Signed-off-by: Michael Burke <michburk@redhat.com>

Author: michburk

roles/discover_latest_image/molecule/default/converge.yml

@@ -90,13 +90,13 @@
           - item.value.hash_algorithm is defined
       loop: "{{ cifmw_discovered_images_dict | dict2items }}"
 
-    - name: Ensure single-image facts are null when no default entry
+    - name: Ensure single-image facts are unchanged
       ansible.builtin.assert:
         that:
-          - cifmw_discovered_image_name is none
-          - cifmw_discovered_image_url is none
-          - cifmw_discovered_hash is none
-          - cifmw_discovered_hash_algorithm is none
+          - "'CentOS-Stream-GenericCloud-x86_64-10' in cifmw_discovered_image_name"
+          - cifmw_discovered_image_url is not none
+          - cifmw_discovered_hash is not none
+          - cifmw_discovered_hash_algorithm is not none
 
 - name: Test multi-image discovery with default entry
   hosts: all

roles/discover_latest_image/tasks/main.yml

@@ -14,13 +14,9 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-- name: Set the discovered image vars to empty/null
+- name: Set the discovered images dict to empty
   ansible.builtin.set_fact:
     cifmw_discovered_images_dict: {}
-    cifmw_discovered_image_name: null
-    cifmw_discovered_image_url: null
-    cifmw_discovered_hash: null
-    cifmw_discovered_hash_algorithm: null
 
 - name: Discover images
   ansible.builtin.include_tasks: per-image.yml