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-sqlite/tests/test_delta_parent_walk.py

@@ -65,6 +65,39 @@ async def test_async_walk_reaches_parent_whatever_the_id_order(
         assert got[CHANNEL] == EXPECTED
 
 
+EMPTY_CHECKPOINT_ID: dict[str, Any] = {
+    "configurable": {**CONFIG["configurable"], "checkpoint_id": ""}
+}
+
+
+def test_sync_empty_checkpoint_id_reads_from_the_latest_checkpoint() -> None:
+    with SqliteSaver.from_conn_string(":memory:") as saver:
+        root = saver.put(CONFIG, _checkpoint("a-older", {CHANNEL: "seed"}), {}, {})
+        saver.put_writes(root, [(CHANNEL, "write-root")], "task")
+        saver.put(root, _checkpoint("z-newer", {}), {}, {})
+
+        got = saver.get_delta_channel_history(
+            config=EMPTY_CHECKPOINT_ID, channels=[CHANNEL]
+        )
+
+        assert got[CHANNEL] == EXPECTED
+
+
+async def test_async_empty_checkpoint_id_reads_from_the_latest_checkpoint() -> None:
+    async with AsyncSqliteSaver.from_conn_string(":memory:") as saver:
+        root = await saver.aput(
+            CONFIG, _checkpoint("a-older", {CHANNEL: "seed"}), {}, {}
+        )
+        await saver.aput_writes(root, [(CHANNEL, "write-root")], "task")
+        await saver.aput(root, _checkpoint("z-newer", {}), {}, {})
+
+        got = await saver.aget_delta_channel_history(
+            config=EMPTY_CHECKPOINT_ID, channels=[CHANNEL]
+        )
+
+        assert got[CHANNEL] == EXPECTED
+
+
 def test_walk_reaches_root_of_long_chain_with_descending_ids() -> None:
     steps = 40
     with SqliteSaver.from_conn_string(":memory:") as saver: