fix crash when user ID attribute is missing from reply

[guillomovitch]

If one of the attributes mentionned in user_id_from_attrs is missing from the IdP answer, SATOSA will crash with the following exception (missing attribute is called sub in this specific case):

[2025-01-10 10:16:08,259] [ERROR] [satosa.proxy_server.__call__] Unknown error
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/satosa/base.py", line 268, in run
    resp = self._run_bound_endpoint(context, spec)
  File "/usr/local/lib/python3.9/site-packages/satosa/base.py", line 193, in _run_bound_endpoint
    return spec(context)
  File "/usr/local/lib/python3.9/site-packages/satosa/backends/saml2.py", line 482, in authn_response
    return self.auth_callback_func(context, self._translate_response(authn_response, context.state))
  File "/usr/local/lib/python3.9/site-packages/satosa/base.py", line 154, in _auth_resp_callback_func
    subject_id = ""
  File "/usr/local/lib/python3.9/site-packages/satosa/base.py", line 155, in <listcomp>
    for attr in self.config["INTERNAL_ATTRIBUTES"]["user_id_from_attrs"]:
KeyError: 'sub'

This PR fixes this issue by ensuring missing attributes are skipped.

[guillomovitch]

Hello. May I have any kind of feedback about this PR, wich seems to be relatively trivial ?