langchain: fix TrajectoryEvalChain.prep_inputs (#19959)

The previous implementation would never be called.

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---------

Co-authored-by: Isaac Francisco <78627776+isahers1@users.noreply.github.com>
Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/langchain/langchain/evaluation/agents/trajectory_eval_chain.py

@@ -283,8 +283,7 @@ The following is the expected answer. Use this to measure correctness:
 
     def prep_inputs(self, inputs: Union[Dict[str, Any], Any]) -> Dict[str, str]:
         """Validate and prep inputs."""
-        if "reference" not in inputs:
-            inputs["reference"] = self._format_reference(inputs.get("reference"))
+        inputs["reference"] = self._format_reference(inputs.get("reference"))
         return super().prep_inputs(inputs)
 
     def _call(