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>

python_repositories/adapters/redis_adapter.py

@@ -91,12 +91,17 @@ class RedisAdapter(JsonRepositoryInterface, ConnectionAwareAdapter):
             return False
 
     def set(self, key: str, data: dict[str, Any]) -> None:
-        """Set a JSON object in Redis."""
+        """Set a JSON object in Redis.
+
+        Accepts any dict, including ``{}``. An empty dict creates a key that
+        ``get()`` returns as ``{}``, not ``None``. Use ``delete()`` to remove a
+        key entirely.
+        """
         # Check input
         if not isinstance(key, str) or len(key) == 0:
             raise ValueError("Key must be a non-empty string")
-        if not isinstance(data, dict) or len(data) == 0:
-            raise ValueError("Data must be a non-empty dictionary")
+        if not isinstance(data, dict):
+            raise ValueError("Data must be a dictionary")
         # Check connection
         self._require_connected()
         assert self._client is not None
@@ -105,7 +110,12 @@ class RedisAdapter(JsonRepositoryInterface, ConnectionAwareAdapter):
         self.logger.debug("Set key", key=key, data_keys=list(data.keys()))
 
     def get(self, key: str) -> dict[str, Any] | None:
-        """Get a JSON object from Redis."""
+        """Get a JSON object from Redis.
+
+        Returns ``None`` if the key is absent. Returns ``{}`` if the key exists
+        with an empty JSON object. Use ``value is not None`` to test existence;
+        avoid truthiness checks (``{}`` is falsy).
+        """
         # Check input
         if not isinstance(key, str) or len(key) == 0:
             raise ValueError("Key must be a non-empty string")