Fix #521: Handle zero-area boxes in compute_iou to prevent ZeroDivisionError

[Armaan-Raisinghani]

Fixes: #521

Changes:

• Added zero-union guard in compute_iou() to return 0.0 instead of crashing
• Added test_compute_iou_zero_area_boxes test covering point boxes, mixed zero/normal boxes, non-overlapping boxes, and perfect overlap

Before:

iou = interArea / float(boxAArea + boxBArea - interArea)  # ZeroDivisionError on point boxes

After:

iou = interArea / union if union > 0 else 0.0

This is consistent with the existing guard in compute_iou_matrix()