Fix conditionals for Ansible 2.19

Ansible 2.19 no longer allows impliction boolean conversions.

See https://ansible.readthedocs.io/projects/ansible-core/devel/porting_guides/porting_guide_core_2.19.html#broken-conditionals

Author: sur5r

roles/unattended_upgrades/tasks/systemd_timers.yml

@@ -46,7 +46,7 @@
     group: 'root'
   when:
     - '_apt_daily.rc == 0'
-    - '_apt_daily_timer_d' # skip if run for the first time in check mode
+    - '_apt_daily_timer_d is defined' # skip if run for the first time in check mode
   notify:
     - 'Restart apt-daily timer'
 
@@ -59,7 +59,7 @@
     group: 'root'
   when:
     - '_apt_daily_upgrade.rc == 0'
-    - '_apt_daily_upgrade_timer_d' # skip if run for the first time in check mode
+    - '_apt_daily_upgrade_timer_d is defined' # skip if run for the first time in check mode
   notify:
     - 'Restart apt-daily-upgrade timer'
 ...