ci/b2c: make initramfs & kernel urls optional

If omitted, the default ones are used.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31570>

.gitlab-ci/b2c/b2c.yml.jinja2.jinja2

@@ -62,7 +62,9 @@ deployment:
               b2c.service="--privileged --tls-verify=false --pid=host docker://{{ '{{' }} fdo_proxy_registry }}/gfx-ci/ci-tron/telegraf:latest" b2c.hostname=dut-{{ '{{' }} machine.full_name }}
               b2c.container="-v {{ '{{' }} job_bucket }}-results:{{ working_dir }} -w {{ working_dir }} {% for mount_volume in mount_volumes %} -v {{ mount_volume }}{% endfor %} --tls-verify=false docker://{{ local_container }} {{ container_cmd }}"
     kernel:
+{% if kernel_url %}
       url: '{{ kernel_url }}'
+{% endif %}
 
       # NOTE: b2c.cache_device should not be here, but this works around
       # a limitation of b2c which will be removed in the next release
@@ -75,8 +77,10 @@ deployment:
         {{ kernel_cmdline_extras }}
         {% endif %}
 
+{% if initramfs_url %}
     initramfs:
       url: '{{ initramfs_url }}'
+{% endif %}
 
 {% if dtb_url %}
     dtb: