Fix SR if SpacingBetweenSlices is not set

`SpacingBetweenSlices` is an optional DICOM tag, so it can not be
present. Only add it to `ImageLibraryEntryDescriptors` if available.

Author: Fedalto

src/highdicom/sr/templates.py

@@ -4015,12 +4015,6 @@ def _generate_cross_sectional_descriptors(
                 relationship_type=RelationshipTypeValues.HAS_ACQ_CONTEXT,
                 unit=codes.UCUM.Millimeter
             ),
-            NumContentItem(
-                name=codes.DCM.SpacingBetweenSlices,
-                value=dataset.SpacingBetweenSlices,
-                relationship_type=RelationshipTypeValues.HAS_ACQ_CONTEXT,
-                unit=codes.UCUM.Millimeter
-            ),
             NumContentItem(
                 name=codes.DCM.SliceThickness,
                 value=dataset.SliceThickness,
@@ -4106,6 +4100,16 @@ def _generate_cross_sectional_descriptors(
                 )
             )
         ]
+        if hasattr(dataset, 'SpacingBetweenSlices'):
+            descriptors.append(
+                NumContentItem(
+                    name=codes.DCM.SpacingBetweenSlices,
+                    value=dataset.SpacingBetweenSlices,
+                    relationship_type=RelationshipTypeValues.HAS_ACQ_CONTEXT,
+                    unit=codes.UCUM.Millimeter
+                ),
+            )
+
         return descriptors
 
     def _is_cross_sectional(self, ds: Dataset) -> bool:

tests/test_sr.py

@@ -5591,6 +5591,11 @@ def test_dx_construction(self):
         assert group[8].name == codes.DCM.ContentTime
         assert group[8].value == content_time
 
+    def test_optional_spacing_between_slices(self):
+        del self._ref_ct_dataset.SpacingBetweenSlices
+        group = ImageLibraryEntryDescriptors(image=self._ref_ct_dataset)
+        assert not group.find(name=codes.DCM.SpacingBetweenSlices)
+
 
 class TestImageLibrary(unittest.TestCase):