Handle empty IMAGE_URL lines in devstack_cache

Handles lines of the form:
        IMAGE_URLS=${IMAGE_URLS:-};;

Change-Id: Id99586393d28c3c11846f76634256cb497812e5e

modules/openstack_project/files/nodepool/scripts/cache_devstack.py

@@ -92,6 +92,8 @@ def local_prep(distribution):
                     line = line[len('${IMAGE_URLS:-'):]
                 if line.endswith('}'):
                     line = line[:-1]
+                if not line:
+                    continue
                 if line[0] == line[-1] == '"':
                     line = line[1:-1]
                 images += [x.strip() for x in line.split(',')]