Refactor Hubble config conditions: use is not none instead of length > 0

Refactor: Replace `length > 0` with `is not none` for Hubble export config

- Addressed reviewer feedback: avoid using `length > 0` which fails on integer types
- Updated Jinja2 conditionals to use `is not none` for proper type safety
- Prevents runtime errors when values are provided as integers (e.g., 100, 5)

Author: ErmolenkoMaxim

roles/network_plugin/cilium/templates/cilium/config.yml.j2

@@ -222,10 +222,10 @@ data:
   hubble-tls-client-ca-files: /var/lib/cilium/tls/hubble/client-ca.crt
 {% endif %}
 
-{% if cilium_hubble_export_file_max_backups is defined and cilium_hubble_export_file_max_backups | length > 0 %}
+{% if cilium_hubble_export_file_max_backups is defined and cilium_hubble_export_file_max_backups is not none %}
   hubble-export-file-max-backups: "{{ cilium_hubble_export_file_max_backups }}"
 {% endif %}
-{% if cilium_hubble_export_file_max_size_mb is defined and cilium_hubble_export_file_max_size_mb | length > 0 %}
+{% if cilium_hubble_export_file_max_size_mb is defined and cilium_hubble_export_file_max_size_mb is not none %}
   hubble-export-file-max-size-mb: "{{ cilium_hubble_export_file_max_size_mb }}"
 {% endif %}