[trainer] fix ignored columns logger (#10219)

* [trainer] fix ignored columns logger

This PR fixes a confusing log entry that says:
```
The following columns in the evaluation set don't have a corresponding argument in `T5ForConditionalGeneration.forward` and have been ignored: .
```
when everything is in order.

* Update src/transformers/trainer.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/trainer.py

@@ -434,9 +434,10 @@ class Trainer:
             self._signature_columns += ["label", "label_ids"]
         columns = [k for k in self._signature_columns if k in dataset.column_names]
         ignored_columns = list(set(dataset.column_names) - set(self._signature_columns))
+        if len(ignored_columns) > 0:
             dset_description = "" if description is None else f"in the {description} set "
             logger.info(
-            f"The following columns {dset_description}don't have a corresponding argument in "
+                f"The following columns {dset_description} don't have a corresponding argument in "
                 f"`{self.model.__class__.__name__}.forward` and have been ignored: {', '.join(ignored_columns)}."
             )