[model cards] Prevent crash on generating widgets if dataset column is empty (#2997)

(or if it has no string columns)

Author: tomaarsen

sentence_transformers/model_card.py

@@ -429,6 +429,9 @@ def set_widget_examples(self, dataset: Dataset | DatasetDict) -> None:
             ]
             str_dataset = dataset[dataset_name].select_columns(columns)
             dataset_size = len(str_dataset)
+            if dataset_size == 0:
+                continue
+
             lengths = {}
             for idx, sample in enumerate(
                 str_dataset.select(random.sample(range(dataset_size), k=min(num_samples_to_check, dataset_size)))