get_chain() function can raise TypeError

Unfortunately I don't have steps to reproduce this consistently. It happened to me when vault units were changing leadership.

Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-vault-1/.venv/lib/python3.8/site-packages/charms/reactive/__init__.py", line 74, in main
    bus.dispatch(restricted=restricted_mode)
  File "/var/lib/juju/agents/unit-vault-1/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 390, in dispatch
    _invoke(other_handlers)
  File "/var/lib/juju/agents/unit-vault-1/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 359, in _invoke
    handler.invoke()
  File "/var/lib/juju/agents/unit-vault-1/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 181, in invoke
    self._action(*args)
  File "/var/lib/juju/agents/unit-vault-1/charm/reactive/vault_handlers.py", line 967, in publish_ca_info
    chain = vault_pki.get_chain()
  File "/var/lib/juju/agents/unit-vault-1/charm/lib/charm/vault_pki.py", line 87, in get_chain
    return client.read('{}/cert/ca_chain'.format(name))['data']['certificate']
TypeError: 'NoneType' object is not subscriptable

I think that either the docstring of the function should be changed to indicate that this exception can be raised by the function (right now the docstring is copy-pasted from is_ca_ready()), or ideally the function should catch the exception and return appropriate value (None, "" ?)