fix: ensure connection check before executing checkpoint in KuzuAdapter (#1268)

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## Description
<!-- Provide a clear description of the changes in this PR -->
kuzu + s3 is failing on dev. What's happening is:

1. During cognee.add, run_pipeline uses
[`run_tasks`](https://github.com/topoteretes/cognee/blob/dev/cognee/modules/pipelines/operations/pipeline.py#L193-L195)
2. run_tasks calls
[`push_to_s3`](https://github.com/topoteretes/cognee/blob/dev/cognee/modules/pipelines/operations/run_tasks.py#L336-L338),
which attempts to
[checkpoint](https://github.com/topoteretes/cognee/blob/dev/cognee/infrastructure/databases/graph/kuzu/adapter.py#L142)
before the push (Kuzu connection is initialized in `cognify`)

Trace:
```
AttributeError: 'NoneType' object has no attribute 'execute'

Traceback (most recent call last):
  File "/Users/daulet/PycharmProjects/cognee-source/examples/python/dynamic_steps_example.py", line 219, in <module>
    loop.run_until_complete(main(steps_to_enable))
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/asyncio/base_events.py", line 687, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/Users/daulet/PycharmProjects/cognee-source/examples/python/dynamic_steps_example.py", line 186, in main
    await cognee.add(text)
  File "/Users/daulet/PycharmProjects/cognee-source/cognee/api/v1/add/add.py", line 145, in add
    async for run_info in cognee_pipeline(
  File "/Users/daulet/PycharmProjects/cognee-source/cognee/modules/pipelines/operations/pipeline.py", line 106, in cognee_pipeline
    async for run_info in run_pipeline(
  File "/Users/daulet/PycharmProjects/cognee-source/cognee/modules/pipelines/operations/pipeline.py", line 197, in run_pipeline
    async for pipeline_run_info in pipeline_run:
  File "/Users/daulet/PycharmProjects/cognee-source/cognee/modules/pipelines/operations/run_tasks.py", line 53, in wrapper
    async for run_info in original_gen(*args, **kwargs):
  File "/Users/daulet/PycharmProjects/cognee-source/cognee/modules/pipelines/operations/run_tasks.py", line 360, in run_tasks
    raise error
  File "/Users/daulet/PycharmProjects/cognee-source/cognee/modules/pipelines/operations/run_tasks.py", line 337, in run_tasks
    await graph_engine.push_to_s3()
  File "/Users/daulet/PycharmProjects/cognee-source/cognee/infrastructure/databases/graph/kuzu/adapter.py", line 142, in push_to_s3
    self.connection.execute("CHECKPOINT;")
```
# After fix

Running `dynamic_steps_example.py`:

### Main branch
```
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### This branch
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```

## DCO Affirmation
I affirm that all code in every commit of this pull request conforms to
the terms of the Topoteretes Developer Certificate of Origin.

cognee/infrastructure/databases/graph/kuzu/adapter.py

@@ -138,8 +138,9 @@ class KuzuAdapter(GraphDBInterface):
 
             s3_file_storage = S3FileStorage("")
 
-            async with self.KUZU_ASYNC_LOCK:
-                self.connection.execute("CHECKPOINT;")
+            if self.connection:
+                async with self.KUZU_ASYNC_LOCK:
+                    self.connection.execute("CHECKPOINT;")
 
             s3_file_storage.s3.put(self.temp_graph_file, self.db_path, recursive=True)