Only write floating ip config when floating_ips are set

The task also ran with an empty list and restarted/enabled ifupdown's networking service; Raspberry Pi OS (NetworkManager) has no /etc/network/interfaces.d at all.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tasks/networking.yml

@@ -7,3 +7,5 @@
     group: root
     mode: 0644
   notify: restart networking service
+  # only ifupdown hosts with floating ips need this (not e.g. Raspberry Pi OS with NetworkManager)
+  when: floating_ips | length > 0