Fix post processor for classification tasks

[alexriedel1]

📝 Description

For classification tasks without ground truth masks, the one class post processor fails on updating the threshold if the model produces an anomaly map while there is no ground truth mask. This PR fixes this

✨ Changes

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• 🐞 Bug fix (non-breaking change which fixes an issue)
• 🔨 Refactor (non-breaking change which refactors the code base)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔒 Security update

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• 🧪 I have written tests that support my changes and prove that my fix is effective or my feature works (if applicable).

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