fix: drop passphrase from CSR task for unencrypted client key

This task file is only included when
`certificate_authority_client_tls_key_passphrase` is empty, so the private key
is created without encryption. Passing the empty passphrase to `openssl_csr`
made the module attempt to decrypt an unencrypted key instead of treating it as
absent. The sibling CSR task for certificates with SANs already omitted the
attribute, so this also aligns both code paths.

Co-authored-by: Copilot <copilot@github.com>

tasks/client_certificate_unprotected.yaml

@@ -16,7 +16,6 @@
     organization_name: "{{ certificate_authority_client_organization_name }}"
     organizational_unit_name: "{{ certificate_authority_client_organizational_unit_name }}"
     path: "{{ certificate_authority_client_path }}/cert-req.pem"
-    privatekey_passphrase: "{{ certificate_authority_client_tls_key_passphrase }}"
     privatekey_path: "{{ certificate_authority_client_path }}/privkey.pem"
     state_or_province_name: "{{ certificate_authority_client_state_or_province_name }}"
   when: |