Fix: skip nested TagSerializer construction when a tag has no children (#14039)

TagSerializer.get_children() built a full nested TagSerializer(many=True)
for every tag, even when it had zero children, likely the common case for
most tags and maybe even most installs. Constructing a DRF ModelSerializer isn't
free (field introspection, deepcopy of declared fields, i18n lookups
all re-run per instantiation), so this scaled GET /api/tags/ linearly
with tag count in pure Python overhead, unrelated to SQL query count.

src/documents/serialisers.py

@@ -674,6 +674,9 @@ class TagSerializer(MatchingModelSerializer, OwnedObjectSerializer):
             ordering = ordering or (Lower("name"),)
             children = children.order_by(*ordering)
 
+        if not children:
+            return []
+
         serializer = TagSerializer(
             children,
             many=True,