Fix source_prefix default value (#26654)

examples/pytorch/summarization/run_summarization.py

@@ -264,7 +264,7 @@ class DataTrainingArguments:
         },
     )
     source_prefix: Optional[str] = field(
-        default="", metadata={"help": "A prefix to add before every source text (useful for T5 models)."}
+        default=None, metadata={"help": "A prefix to add before every source text (useful for T5 models)."}
     )
 
     forced_bos_token: Optional[str] = field(