fix: use truthiness check for asyncConfig parsing

Revert to truthiness check when parsing asyncConfig from server
response. An empty dict {} means no custom config and should be
treated as absent (None), not as an _AsyncReplicationConfig with
all None fields.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jfrancoa

weaviate/collections/classes/config_methods.py

@@ -398,7 +398,7 @@ def _collection_config_from_json(schema: Dict[str, Any]) -> _CollectionConfig:
                     propagation_concurrency=async_cfg.get("propagationConcurrency"),
                     propagation_batch_size=async_cfg.get("propagationBatchSize"),
                 )
-                if (async_cfg := schema["replicationConfig"].get("asyncConfig")) is not None
+                if (async_cfg := schema["replicationConfig"].get("asyncConfig"))
                 else None
             ),
         ),