chore: fix the qlib *hypothesis_and_feedback

when access in the prompt:

rdagent/scenarios/qlib/prompts.yaml:298:
{% if last_hypothesis_and_feedback != "" %}
rdagent/components/proposal/prompts.yaml:17:
{% if hypothesis_and_feedback|length == 0 %}

Author: inevity

rdagent/scenarios/qlib/proposal/factor_proposal.py

@@ -22,14 +22,15 @@ def prepare_context(self, trace: Trace) -> Tuple[dict, bool]:
                 trace=trace,
             )
             if len(trace.hist) > 0
-            else "No previous hypothesis and feedback available since it's the first round."
+            else str()
+
         )
         last_hypothesis_and_feedback = (
             T("scenarios.qlib.prompts:last_hypothesis_and_feedback").r(
                 experiment=trace.hist[-1][0], feedback=trace.hist[-1][1]
             )
             if len(trace.hist) > 0
-            else "No previous hypothesis and feedback available since it's the first round."
+            else str()
         )
 
         context_dict = {