fix: do not skip unit config when `systemd_unit_config` contains undefined variable, fail instead

defaults/main.yml

@@ -1,3 +1,5 @@
-systemd_unit_scope: system # alternative: 'global' (all users)
+---
+systemd_unit_scope: system  # alternative: 'global' (all users)
+systemd_unit_config: null
 systemd_unit_dropin_config_name: override
-systemd_unit_restart_if_active: no
+systemd_unit_restart_if_active: false

tasks/main.yml

@@ -19,7 +19,7 @@
            "user" if systemd_unit_scope == "global" else systemd_unit_scope
          ) }}
   register: _config_file
-  when: systemd_unit_config is defined
+  when: systemd_unit_config
 - name: 'create parent folder for drop-in config files of systemd unit {{ systemd_unit_name }}'
   file:
     path: '{{ _config_dir_path }}/{{ systemd_unit_name }}.d'