fix: faithfulness for unrelated answering

src/ragas/metrics/_faithfulness.py

@@ -96,6 +96,17 @@
     }}
 ]
 
+Context:
+Albert Einstein was a German-born theoretical physicist who is widely held to be one of the greatest and most influential scientists of all time.
+statement_1: Nil
+Answer:
+[
+     {{
+        "statement_1": "Nil",
+        "reason": "The statement is invalid",
+        "verdict": "No"
+    }}
+]
 
 
 context:
@@ -138,6 +149,7 @@ def _score_batch(
             prompts = []
             for context, output in zip(contexts, result.generations):
                 statements = load_as_json(output[0].text).get("statements", [])
+                statements = statements if statements != [] else ["Nil"]
                 statements_str: str = "\n".join(
                     [f"statement_{i+1}: {st}" for i, st in enumerate(statements)]
                 )