Handle empty runtime_images

If there is no demand for docker images to be inserted into nexus
during runtime, runtime_images might be empty and this will
cause unhandled error with trying to insert undefined images

Change-Id: I77147b64a3d288922eec5db7e7ce422ba2ed8c12
Signed-off-by: Michal Ptacek <m.ptacek@partner.samsung.com>
Issue-ID: OOM-1587
diff --git a/ansible/infrastructure.yml b/ansible/infrastructure.yml
index 709a351..0866733 100644
--- a/ansible/infrastructure.yml
+++ b/ansible/infrastructure.yml
@@ -61,7 +61,7 @@
     - role: nexus
       vars:
         phase: runtime-populate
-      when: runtime_images is defined
+      when: runtime_images is defined and runtime_images is not none
 
 - name: Setup base for Kubernetes nodes
   hosts: kubernetes