Allow empty payloads in Redis and MinIO adapters.

Relax write validation so {} and zero-byte BytesIO round-trip correctly,
document None-vs-empty semantics on interfaces and adapters, and add
integration tests for placeholders and existence distinction.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/integration/minio/minio_adapter_test.py

@@ -328,6 +328,44 @@ def test_should_raise_value_error_on_invalid_put_data(
             minio_adapter.put(object_name, invalid)  # type: ignore
 
 
+def test_should_put_and_get_empty_bytesio(
+    minio_adapter: MinioAdapter,
+    minio_config: MinioConfig,
+) -> None:
+    """Test that the MinioAdapter can put and get a zero-byte object."""
+    object_name = "empty_object"
+    empty_data = BytesIO()
+    assert minio_adapter.get(object_name) is None
+    minio_adapter.put(object_name, empty_data)
+    received_data = minio_adapter.get(object_name)
+    assert received_data is not None
+    assert same_data(empty_data, received_data)
+    minio_adapter._client.remove_object(minio_config.bucket, object_name)  # type: ignore[union-attr]
+
+
+def test_should_distinguish_missing_object_from_empty_object(
+    minio_adapter: MinioAdapter,
+    minio_config: MinioConfig,
+) -> None:
+    """Test that missing objects and zero-byte objects are distinguishable."""
+    object_name = "empty_object"
+    minio_adapter.put(object_name, BytesIO())
+    assert minio_adapter.get("other_object") is None
+    minio_adapter.delete(object_name)
+    assert minio_adapter.get(object_name) is None
+
+
+def test_should_list_zero_byte_object(
+    minio_adapter: MinioAdapter,
+    minio_config: MinioConfig,
+) -> None:
+    """Test that a zero-byte object appears in object listings."""
+    object_name = "empty_object"
+    minio_adapter.put(object_name, BytesIO())
+    assert object_name in minio_adapter.list_objects()
+    minio_adapter._client.remove_object(minio_config.bucket, object_name)  # type: ignore[union-attr]
+
+
 def test_should_raise_value_error_on_invalid_put_content_type(
     data: BytesIO,
     minio_adapter: MinioAdapter,