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 <[email protected]>

python_repositories/adapters/minio_adapter.py

@@ -114,12 +114,17 @@ class MinioAdapter(ObjectRepositoryInterface, ConnectionAwareAdapter):
         data: BytesIO,
         content_type: str = "application/octet-stream",
     ) -> None:
-        """Put an object into the Minio bucket."""
+        """Put an object into the Minio bucket.
+
+        Accepts zero-byte ``BytesIO``. A zero-byte object is returned by
+        ``get()`` as an empty buffer, not ``None``. Use ``delete()`` to remove
+        an object entirely.
+        """
         # Check input
         if not isinstance(object_name, str) or len(object_name) == 0:
             raise ValueError("object_name must be a non-empty string")
-        if not isinstance(data, BytesIO) or data.getbuffer().nbytes == 0:
-            raise ValueError("data must be a non-empty BytesIO object")
+        if not isinstance(data, BytesIO):
+            raise ValueError("data must be a BytesIO object")
         if not isinstance(content_type, str) or len(content_type) == 0:
             raise ValueError("content_type must be a non-empty string")
         # Check connection
@@ -143,7 +148,12 @@ class MinioAdapter(ObjectRepositoryInterface, ConnectionAwareAdapter):
         )
 
     def get(self, object_name: str) -> BytesIO | None:
-        """Get an object from the Minio bucket."""
+        """Get an object from the Minio bucket.
+
+        Returns ``None`` when the object does not exist. Returns an empty
+        ``BytesIO`` for a zero-byte object. Use ``value is not None`` to test
+        existence.
+        """
         # Check input
         if not isinstance(object_name, str) or len(object_name) == 0:
             raise ValueError("object_name must be a non-empty string")