generator: handle server specs with no variables

[brutasse]

Fix for:

==================================== ERRORS ====================================
________________ ERROR collecting tests/test_partner_client.py _________________
tests/test_partner_client.py:12: in <module>
    from exoscale.api.partner import Client
exoscale/api/partner.py:17: in <module>
    BasePartnerClient = create_client_class(partner_api_spec)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
exoscale/api/generator.py:349: in create_client_class
    "__doc__": _client_docstring(api_spec),
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^
exoscale/api/generator.py:303: in _client_docstring
    for name, variable in server["variables"].items():
                          ^^^^^^^^^^^^^^^^^^^
E   KeyError: 'variables'
=========================== short test summary info ============================
ERROR tests/test_partner_client.py - KeyError: 'variables'
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!
=============================== 1 error in 0.23s ===============================
Error: Process completed with exit code 2.
``