fix(vaultwarden): allow vaultwarden_container_labels to not be set

[transcaffeine]

Fixes:

TASK [famedly.services.vaultwarden : Ensure vaultwarden container image is running] ***********************************
fatal: [hostname]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: {{ vaultwarden_container_labels_base | combine(vaultwarden_container_labels, recursive=True) }}: 'vaultwarden_container_labels' is undefined. 'vaultwarden_container_labels' is undefined. {{ vaultwarden_container_labels_base | combine(vaultwarden_container_labels, recursive=True) }}: 'vaultwarden_container_labels' is undefined. 'vaultwarden_container_labels' is undefined\n\nThe error appears to be in '/home/user/git/project/ansible_collections/famedly/services/roles/vaultwarden/tasks/main.yml': line 76, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n- name: Ensure vaultwarden container image is running\n  ^ here\n"}

See #36

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All committers have signed the CLA.

[transcaffeine]

@jcgruenhage could i get a review on this PR?

[transcaffeine]

@lrsksr this seems stale, could i get feedback and/or a review?