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]>

tests/integration/redis/redis_adapter_test.py

@@ -190,12 +190,45 @@ def test_should_raise_value_error_on_invalid_set_data(
 ) -> None:
     """Test that the RedisAdapter raises ValueError when setting with invalid data."""
     key = "test_key"
-    invalid_data = ["", 123, None, [], {}]
+    invalid_data = ["", 123, None, []]
     for data in invalid_data:
         with pytest.raises(ValueError):
             redis_adapter.set(key, data)  # type: ignore
 
 
+def test_should_set_and_get_empty_dict(
+    redis_adapter: RedisAdapter,
+) -> None:
+    """Test that the RedisAdapter can set and get an empty dict."""
+    key = "empty_key"
+    assert redis_adapter.get(key) is None
+    redis_adapter.set(key, {})
+    value = redis_adapter.get(key)
+    assert value is not None
+    assert value == {}
+
+
+def test_should_distinguish_missing_key_from_empty_dict(
+    redis_adapter: RedisAdapter,
+) -> None:
+    """Test that missing keys and empty dicts are distinguishable."""
+    key = "empty_key"
+    redis_adapter.set(key, {})
+    assert redis_adapter.get("other_key") is None
+    redis_adapter.delete(key)
+    assert redis_adapter.get(key) is None
+
+
+def test_should_list_empty_dict_key(
+    redis_adapter: RedisAdapter,
+) -> None:
+    """Test that a key with an empty dict appears in key listings."""
+    key = "empty_key"
+    redis_adapter.set(key, {})
+    assert key in redis_adapter.list_keys(key)
+    assert key in list(redis_adapter.scan_keys(key))
+
+
 def test_should_raise_connection_error_on_set_when_not_connected(
     redis_config: RedisConfig,
     data: dict[str, str],