Workaround failures when project is unset

In cases, when we want to have only domain scope, we set project to
an empty string or null.

Needed-By: https://review.opendev.org/c/openstack/openstack-ansible-os_heat/+/879963
Change-Id: Iac723a4e748dc1a0c3769934e4ec73019e308aea

roles/service_setup/tasks/main.yml

@@ -66,13 +66,15 @@
       delay: 10
 
     - name: Add service users
+      vars:
+        default_project: "{{ item.project | default(_service_project_name) }}"
       openstack.cloud.identity_user:
         cloud: default
         state: present
         name: "{{ item.name }}"
         password: "{{ item.password }}"
         domain: "{{ item.domain | default('default') }}"
-        default_project: "{{ item.project | default(_service_project_name) }}"
+        default_project: "{{ (default_project is truthy) | ternary(default_project, omit) }}"
         endpoint_type: admin
         validate_certs: "{{ not (_service_adminuri_insecure | default(True) | bool) }}"
         update_password: always