generate-certificate output should be more explicit

I tried to generate a certificate with the following command:

$ juju run-action --wait vault/1 generate-certificate common-name=test
unit-vault-1:
  UnitId: vault/1
  id: "2304"
  message: 'AttributeError: ''NoneType'' object has no attribute ''split'''
  results: {}
  status: failed
  timing:
    completed: 2024-07-09 10:04:37 +0000 UTC
    enqueued: 2024-07-09 10:04:36 +0000 UTC
    started: 2024-07-09 10:04:36 +0000 UTC

I know now that I need to also supply `sans` as well, but that should be more clear from the message. I had to check the debug log to understand that I was missing sans:
[...]
  File "/var/lib/juju/agents/unit-vault-1/charm/actions/generate-certificate", line 262, in generate_cert
    sans=list(sans_list.split()),
AttributeError: 'NoneType' object has no attribute 'split'

Version:
juju controller: 2.9.49
charm: 1.8.8, 1.8/stable, rev 108