fix snapraid conditional

roles/snapraid/tasks/installation.yml

@@ -4,8 +4,8 @@
     name: xfsprogs
     state: present
   when:
-    -  (snapraid_data_disks   | selectattr('fs', '==', 'xfs') | list | length)|int > 0 or
-       (snapraid_parity_disks | selectattr('fs', '==', 'xfs') | list | length)|int > 0
+    -  (snapraid_data_disks   | selectattr('fs', 'defined') | selectattr('fs', '==', 'xfs') | list | length)|int > 0 or
+       (snapraid_parity_disks | selectattr('fs', 'defined') | selectattr('fs', '==', 'xfs') | list | length)|int > 0
 
 - name: Install snapraid
   package: