fix: skip empty face clusters in default_target_face (#1757)

Skip face clusters when no best face was detected, preventing a NoneType error while preserving normal face-detection behavior.

Closes #1755

modules/face_analyser.py

@@ -335,6 +335,9 @@ def default_target_face():
                 best_frame = frame
                 break
 
+        if best_face is None:
+            continue  # No faces detected in this cluster — skip
+
         for frame in map['target_faces_in_frame']:
             for face in frame['faces']:
                 if face['det_score'] > best_face['det_score']: