Bug 2088817

Description of problem:
I've recently upgraded AlmaLinux from 8.5 to 8.6.
I am using pam_sss_gss together with sudo.
This setup ceased to work under certain circumstances:
- it does NOT work when logged in using sss with gssapi-keyex authentication
- it works when logged in on Linux console (both text and sddm work).

Version-Release number of selected component (if applicable):
sssd-client-2.6.2-4.el8_6.x86_64

How reproducible:
Always, as described above.

Steps to Reproduce:
1. Log in using ssh and GSSAPI authentication to IPA client system.
2. Add 'auth sufficient pam_sss_gss.so' as the first line to /etc/pam.d/sudo and configure sssd.conf for the above.
3. Invoke 'sudo -l'. It asks for password while it should succeed with sss gss authentication.
4. When debug is enabled the following is printed:
pam_sss_gss: sss_cli_getenv() call failed [2]: No such file or directory

Actual results:
SSSD GSS authentication fails. sudo prompts for password.

Expected results:
SSSD GSS authentication succeeds. No password prompt.

Additional info:
This is caused by introduction of 'sss_cli_getenv' function in pam_sss_gss.c file and mishandling the NULL result from 'getenv' function. NULL is perfectly valid and SHOULD NOT result in error (ENOENT) returned by this function, especially that this case is properly handled by previous version of the module where default cache is picked (from sssd-client-2.5.2).

Please note the comment above call to 'sss_cli_getenv':
/* Get non-default ccache if specified, may be NULL. */
clearly indicates that the requested environment variable is OPTIONAL. Please handle it as such.

The same issue exists on Fedora 35.