fix(checkpoint-postgres,checkpoint-sqlite): treat an empty checkpoint_id as the latest in delta history (#9262)

`PostgresSaver` and `SqliteSaver`, sync and async, read a DeltaChannel's
history from the latest checkpoint only when the config's
`checkpoint_id` was missing or `None`. An empty one was looked up as an
id, matched nothing, and came back with no seed and no writes, while
their own `get_tuple` treats an empty id as the latest checkpoint, as
the default implementation does. They now check for a missing or empty
id the way `get_tuple` does. The in-memory saver gets the same in #9257.

Graphs always pass the id of a saved checkpoint, so only direct callers
of the saver API hit this.

## Tests

-
`test_{sync,async}_empty_checkpoint_id_reads_from_the_latest_checkpoint`
in `checkpoint-postgres/tests/test_delta_pagination.py` and
`checkpoint-sqlite/tests/test_delta_parent_walk.py`: all four fail on
main.
- The `checkpoint-postgres` (Postgres 16) and `checkpoint-sqlite` suites
pass, and `lint_package` and `lint_tests` are clean in both.

libs/checkpoint-postgres/tests/test_delta_pagination.py

@@ -164,6 +164,40 @@ def test_sync_walk_reads_nothing_newer_than_the_target(
     assert read == _ids_from_target_down(configs)
 
 
+def _empty_checkpoint_id(config: dict) -> dict:
+    return {"configurable": {**config["configurable"], "checkpoint_id": ""}}
+
+
+async def test_async_empty_checkpoint_id_reads_from_the_latest_checkpoint() -> None:
+    async with AsyncPostgresSaver.from_conn_string(DEFAULT_URI) as saver:
+        await saver.setup()
+        configs = await _abuild_chain(saver)
+        latest = await saver.aget_delta_channel_history(
+            config=configs[-1], channels=[CHANNEL]
+        )
+
+        result = await saver.aget_delta_channel_history(
+            config=_empty_checkpoint_id(configs[-1]), channels=[CHANNEL]
+        )
+
+    assert latest[CHANNEL]["writes"]
+    assert result == latest
+
+
+def test_sync_empty_checkpoint_id_reads_from_the_latest_checkpoint() -> None:
+    with PostgresSaver.from_conn_string(DEFAULT_URI) as saver:
+        saver.setup()
+        configs = _build_chain(saver)
+        latest = saver.get_delta_channel_history(config=configs[-1], channels=[CHANNEL])
+
+        result = saver.get_delta_channel_history(
+            config=_empty_checkpoint_id(configs[-1]), channels=[CHANNEL]
+        )
+
+    assert latest[CHANNEL]["writes"]
+    assert result == latest
+
+
 async def test_root_target_has_no_history_and_still_terminates(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None: