rest: Check that ca_cert_data is not None

[JohnAZoidberg]

The default value of this variable is None but you can't check foo in None:

root@9f485191af50:/# /redfish/upload_firmware_ilo_repository.py 
Traceback (most recent call last):
  File "/redfish/upload_firmware_ilo_repository.py", line 103, in <module>
    REDFISHOBJ = RedfishClient(base_url=SYSTEM_URL, username=LOGIN_ACCOUNT, \
  File "/usr/local/lib/python3.9/site-packages/redfish/rest/v1.py", line 481, in __init__
    super(RedfishClient, self).__init__(default_prefix='/redfish/v1/', is_redfish=True,
  File "/usr/local/lib/python3.9/site-packages/redfish/rest/v1.py", line 211, in __init__
    self.auth_type = self._get_auth_type(auth, ca_cert_data=ca_cert_data, **client_kwargs)
  File "/usr/local/lib/python3.9/site-packages/redfish/rest/v1.py", line 232, in _get_auth_type
    if 'cert_file' in ca_cert_data and ca_cert_data['cert_file']:
TypeError: argument of type 'NoneType' is not iterable

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