NGO slug validator handling of Anonymous User

[sentry-io[bot]]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: backend/donations/views/api.py

Function	Unhandled Issue
validate_ngo_slug	AttributeError: 'AnonymousUser' object has no attribute 'ngo' /api/ngo/check-u... Event Count: 1

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