test: add edge-case tests for empty annotation comparisons

Cover three boundary scenarios that previously raised ZeroDivisionError:
- Empty reference, non-empty test → sensitivity=NaN, PPV=0.0
- Non-empty reference, empty test → sensitivity=0.0, PPV=NaN
- Both empty → both NaN

Author: haoyu-haoyu

tests/test_processing.py

@@ -199,3 +199,33 @@ def test_xqrs(self):
 
         assert comparitor.sensitivity > 0.99
         assert comparitor.positive_predictivity > 0.99
+
+    def test_compare_annotations_empty_ref(self):
+        """When reference annotations are empty, sensitivity should be NaN."""
+        import math
+
+        comparitor = processing.compare_annotations(
+            np.array([]), np.array([10, 20, 30]), 5
+        )
+        assert math.isnan(comparitor.sensitivity)
+        assert comparitor.positive_predictivity == 0.0
+
+    def test_compare_annotations_empty_test(self):
+        """When test annotations are empty, PPV should be NaN."""
+        import math
+
+        comparitor = processing.compare_annotations(
+            np.array([10, 20, 30]), np.array([]), 5
+        )
+        assert comparitor.sensitivity == 0.0
+        assert math.isnan(comparitor.positive_predictivity)
+
+    def test_compare_annotations_both_empty(self):
+        """When both annotation arrays are empty, both metrics should be NaN."""
+        import math
+
+        comparitor = processing.compare_annotations(
+            np.array([]), np.array([]), 5
+        )
+        assert math.isnan(comparitor.sensitivity)
+        assert math.isnan(comparitor.positive_predictivity)