Allow empty label on Ultralytics YOLO dataset (#1264)

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### Summary

In
[Ultralytics](https://docs.ultralytics.com/yolov5/tutorials/tips_for_best_training_results/#dataset:~:text=Background%20images.%20Background%20images%20are%20images%20with%20no%20objects%20that%20are%20added%20to%20a%20dataset%20to%20reduce%20False%20Positives%20(FP).%20We%20recommend%20about%200%2D10%25%20background%20images%20to%20help%20reduce%20FPs%20(COCO%20has%201000%20background%20images%20for%20reference%2C%201%25%20of%20the%20total).%20No%20labels%20are%20required%20for%20background%20images.)
YOLO dataset, there is no label required for the background images. But
in `datumaro`, the `_check_ann_file_impl` of the `_YoloLooseImporter`
class will filter all empty label `.txt` and does not search for images
without label `.txt`. This PR will not search for images without label
`.txt` but will add images with empty label `.txt` to the dataset.

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Author: imyhxy

src/datumaro/plugins/data_formats/yolo/importer.py

@@ -160,6 +160,15 @@ class _YoloUltralyticsImporter(_YoloLooseImporter):
     META_FILE = YoloUltralyticsPath.META_FILE
     FORMAT = YoloFormatType.yolo_ultralytics.name
 
+    @classmethod
+    def _check_ann_file_impl(cls, fp: TextIOWrapper) -> bool:
+        try:
+            return _YoloLooseImporter._check_ann_file_impl(fp)
+        except DatasetImportError as e:
+            if e.args[0] == "Empty file is not allowed.":
+                return True
+            raise
+
 
 class YoloImporter(Importer):
     SUB_IMPORTERS: Dict[YoloFormatType, Importer] = {