BUG:Potential bugs and robustness issues in FaceDetector.detect_faces

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

FaceDetector.detect_faces can produce inconsistent or unsafe results: class_ids are returned unfiltered while boxes and embeddings are filtered by confidence, cropped face images can be empty (leading to possible crashes), and ids is returned as a string instead of a list, making downstream usage unreliable.

**Prposed Solution**
Filter class_ids in the same way as boxes and scores, skip empty face crops before preprocessing, return ids as a proper list instead of a string, and add an explicit early return when no faces are detected to make behavior clearer and safer.
I had like to implement the solution.

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