Modify approve CSRs task changed_when, failed_when

The changed_when conditional has been modified to use the length Jinja
filter as this is what's used elsewhere in the codebase when having a
boolean expression for an empty string.

The failed_when conditional has been removed because I want this task to
fail when the command exits with a non-zero exit code. Conveniently,
this is already how failed_when is defined by default for the
ansible.builtin.command task module. I derived this through trial and
error, plus ansible's source code.

playbooks/k8s_post_controllers.yml

@@ -92,8 +92,7 @@
 
         - name: Approves CSRs that are pending from nodes' kubelets
           ansible.builtin.command: "{{ sign_kubelet_csrs_script_path }}"
-          changed_when: approve_csrs_script.stdout != ""
-          failed_when: approve_csrs_script.stderr != ""
+          changed_when: approve_csrs_script.stdout | length != 0
           become: true
           register: approve_csrs_script