Set the default for ssh_public_key to empty list.

ansible/playbooks/single_user_management.yml

@@ -38,7 +38,7 @@
         sn: "{{ last_name }}"
         mail: "{{ email }}"
         loginshell: "{{ login_shell | default('/bin/bash') }}"
-        sshpubkey: "{{ ssh_public_key }}"
+        sshpubkey: "{{ ssh_public_key | default([]) }}"
         validate_certs: false
         state: "{{ desired_state | default('present') }}"
       when: username is defined