Use more idiomatic way of checking for variable presence

roles/borg-client/defaults/main.yml

 ---
 backup_host: "borg@vostok.archlinux.org"
 backup_dir: "/backup/{{inventory_hostname}}"
-postgres_backup_dir: ''
-mysql_backup_dir: ''
-mysql_backup_defaults: ''

roles/borg-client/tasks/main.yml

@@ -23,7 +23,7 @@
 
 - name: install postgres backup script
   template: src=backup-postgres.sh.j2 dest=/usr/local/bin/backup-postgres.sh owner=root group=root mode=0755
-  when: postgres_backup_dir != ""
+  when: postgres_backup_dir is defined
 
 - name: check whether postgres user exists
   command: getent passwd postgres
@@ -32,19 +32,19 @@
 
 - name: make postgres backup directory
   file: path=/var/lib/postgres/backup owner=postgres group=postgres state=directory
-  when: check_postgres_user|succeeded and postgres_backup_dir != ""
+  when: check_postgres_user|succeeded and postgres_backup_dir is defined
 
 - name: install mysql backup script
   template: src=backup-mysql.sh.j2 dest=/usr/local/bin/backup-mysql.sh owner=root group=root mode=0755
-  when: mysql_backup_dir != ""
+  when: mysql_backup_dir is defined
 
 - name: install mysql backup config
   template: src=backup-my.cnf.j2 dest={{mysql_backup_defaults}}
-  when: mysql_backup_defaults != ""
+  when: mysql_backup_defaults is defined
 
 - name: create mysql backup directory
   file: path={{mysql_backup_dir}} state=directory owner=root group=root
-  when: mysql_backup_dir != ""
+  when: mysql_backup_dir is defined
 
 - name: install systemd timers for backup
   copy: src={{ item }} dest=/etc/systemd/system/{{ item }} owner=root group=root mode=0644

roles/matrix/tasks/main.yml

@@ -2,7 +2,7 @@
 
 - name: create ssl cert
   command: certbot certonly --email webmaster@archlinux.org --agree-tos --rsa-key-size 4096 --renew-by-default --webroot -w {{letsencrypt_validation_dir}} -d '{{ matrix_domain }}' creates='/etc/letsencrypt/live/{{ matrix_domain }}/fullchain.pem'
-  when: 'matrix_domain != ""'
+  when: 'matrix_domain is defined'
 
 - name: install packages
   pacman: name=python2-virtualenv,git,npm
@@ -133,7 +133,7 @@
   template: src=nginx.d.conf.j2 dest=/etc/nginx/nginx.d/matrix.conf owner=root group=root mode=0644
   notify:
     - reload nginx
-  when: 'matrix_domain != ""'
+  when: 'matrix_domain is defined'
 
 - name: install matrix units
   copy: src={{ item }} dest=/etc/systemd/system/{{ item }} owner=root group=root mode=0644

roles/syncrepo/tasks/main.yml

@@ -2,7 +2,7 @@
 
 - name: create ssl cert
   command: certbot certonly --email webmaster@archlinux.org --agree-tos --rsa-key-size 4096 --renew-by-default --webroot -w {{letsencrypt_validation_dir}} -d '{{ mirror_domain }}' creates='/etc/letsencrypt/live/{{ mirror_domain }}/fullchain.pem'
-  when: 'mirror_domain != ""'
+  when: 'mirror_domain is defined'
 
 - name: install rsync
   pacman: name=rsync state=present
@@ -48,4 +48,4 @@
   template: src=nginx.d.conf.j2 dest=/etc/nginx/nginx.d/syncrepo.conf owner=root group=root mode=0644
   notify:
     - reload nginx
-  when: 'mirror_domain != ""'
+  when: 'mirror_domain is defined'