[change] Set default site to empty instead of removing it

This should avoid the default site to be overridden
when upgrading the Freeradius package, which can cause
the openwisp site to break.

tasks/freeradius.yml

@@ -174,9 +174,11 @@
     group: freerad
 
 - name: Remove default site
-  file:
+  # Leave empty to ensure upgrades to freeradius
+  # will not overwrite it
+  copy:
+    content: "# Empty, Generated by Ansible OpenWISP\n\n"
     dest: "{{ item }}"
-    state: absent
   with_items:
     - "{{ freeradius_sites_enabled_dir }}/default"