fix: remove placeholder issue URL from test docstring

The XXX placeholder was never updated with a real issue number.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

tests/integration/test_single_node.py

@@ -247,7 +247,7 @@ async def test_null_boolean_and_datetime(self, cluster_address: str) -> None:
 
         This test catches a dqlite bug where NULL values in BOOLEAN and DATETIME
         columns are incorrectly returned as their zero values (False/empty string)
-        instead of NULL. See: https://github.com/canonical/dqlite/issues/XXX
+        instead of NULL.
         """
         async with await connect(cluster_address) as conn:
             await conn.execute("DROP TABLE IF EXISTS test_null_special")