Fix error in throttling when request.user is None

[4nickel]

While trying to remove the Django authentication module, I've ran into an error when using the configuration below.
Specifically, an AttributeError is raised because DRF does not expect the user to be None.

# settings.py
REST_FRAMEWORK = {
    ...
    "DEFAULT_THROTTLE_CLASSES": (
        "rest_framework.throttling.AnonRateThrottle",
    ),
    "DEFAULT_THROTTLE_RATES": {"anon": "10000/day"},
    'DEFAULT_AUTHENTICATION_CLASSES': [],
    'DEFAULT_PERMISSION_CLASSES': [],
    'UNAUTHENTICATED_USER': None,
}

This PR fixes the issue by checking if request.user is set before proceeding with further authentication checks.
The test suite and pre-commit hooks are passing. If needed, I'll be happy to write a test-case for this.

As this is my first contribution to DRF please let me know in case I am missing any contribution guidelines. I'd like for the process to be as seamless as possible.

Thank you for your time and consideration.

[shredding]

This affects me as well. Is there a chance to merge that soon?

[alexgutjahr]

+1

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[tomchristie]

Normally you'd expect an UnauthenticatedUser instance in this case, but I don't have any objections to this.