Fix bedrock llm boto3 client instantiation (langchain-ai#5629)

Same issue as langchain-ai#5574

Author: Sean Morgan

langchain/llms/bedrock.py

@@ -101,7 +101,7 @@ def validate_environment(cls, values: Dict) -> Dict:
         """Validate that AWS credentials to and python package exists in environment."""
 
         # Skip creating new client if passed in constructor
-        if "client" in values:
+        if values["client"] is not None:
             return values
 
         try: