Fixes KeyError in AmazonKendraRetriever initializer (#7464)

### Description
argument variable client is marked as required in commit
81e5b1ad36 which breaks the default way of
initialization providing only index_id. This commit avoid KeyError
exception when it is initialized without a client variable
### Dependencies
no dependency required

langchain/retrievers/kendra.py

@@ -207,7 +207,7 @@ class AmazonKendraRetriever(BaseRetriever):
 
     @root_validator(pre=True)
     def create_client(cls, values: Dict[str, Any]) -> Dict[str, Any]:
-        if values["client"] is not None:
+        if values.get("client") is not None:
             return values
 
         try: