Fix exception when run export for an empty task (#5396)

Fix https://github.com/opencv/cvat/issues/5245

The PR contains a simple fix. Just return BAD REQUEST if somebody tries
to export a task without data. It doesn't make sense. But a more complex
fix will require changing a massive amount of code. It doesn't make any
sense to support such a weird scenario.

CHANGELOG.md

@@ -84,6 +84,7 @@ non-ascii paths while adding files from "Connected file share" (issue #4428)
 - Visibility and ignored information fail to be loaded (MOT dataset format) (<https://github.com/opencv/cvat/pull/5270>)
 - Added force logout on CVAT app start if token is missing (<https://github.com/opencv/cvat/pull/5331>)
 - Missed token with using social account authentication (<https://github.com/opencv/cvat/pull/5344>)
+- An exception when run export for an empty task (<https://github.com/opencv/cvat/pull/5396>)
 - Fixed FBRS serverless function runtime error on images with alpha channel (<https://github.com/opencv/cvat/pull/5384>)
 - Attaching manifest with custom name (<https://github.com/opencv/cvat/pull/5377>)
 - Uploading non-zip annotaion files (<https://github.com/opencv/cvat/pull/5386>)

cvat/apps/engine/views.py

@@ -1061,6 +1061,7 @@ class TaskViewSet(viewsets.GenericViewSet, mixins.ListModelMixin,
             ), description='Download of file started'),
             '201': OpenApiResponse(description='Annotations file is ready to download'),
             '202': OpenApiResponse(description='Dump of annotations has been started'),
+            '400': OpenApiResponse(description='Exporting without data is not allowed'),
             '405': OpenApiResponse(description='Format is not available'),
         })
     @extend_schema(methods=['PUT'], summary='Method allows to upload task annotations',
@@ -1120,14 +1121,18 @@ class TaskViewSet(viewsets.GenericViewSet, mixins.ListModelMixin,
     def annotations(self, request, pk):
         self._object = self.get_object() # force to call check_object_permissions
         if request.method == 'GET':
-            return self.export_annotations(
-                request=request,
-                pk=pk,
-                db_obj=self._object,
-                export_func=_export_annotations,
-                callback=dm.views.export_task_annotations,
-                get_data=dm.task.get_task_data,
-            )
+            if self._object.data:
+                return self.export_annotations(
+                    request=request,
+                    pk=pk,
+                    db_obj=self._object,
+                    export_func=_export_annotations,
+                    callback=dm.views.export_task_annotations,
+                    get_data=dm.task.get_task_data,
+                )
+            else:
+                return Response(data="Exporting a task without data is not allowed",
+                    status=status.HTTP_400_BAD_REQUEST)
         elif request.method == 'POST' or request.method == 'OPTIONS':
             return self.import_annotations(
                 request=request,
@@ -1290,6 +1295,7 @@ class TaskViewSet(viewsets.GenericViewSet, mixins.ListModelMixin,
             '200': OpenApiResponse(OpenApiTypes.BINARY, description='Download of file started'),
             '201': OpenApiResponse(description='Output file is ready for downloading'),
             '202': OpenApiResponse(description='Exporting has been started'),
+            '400': OpenApiResponse(description='Exporting without data is not allowed'),
             '405': OpenApiResponse(description='Format is not available'),
         })
     @action(detail=True, methods=['GET'], serializer_class=None,
@@ -1297,13 +1303,16 @@ class TaskViewSet(viewsets.GenericViewSet, mixins.ListModelMixin,
     def dataset_export(self, request, pk):
         self._object = self.get_object() # force to call check_object_permissions
 
-        return self.export_annotations(
-            request=request,
-            pk=pk,
-            db_obj=self._object,
-            export_func=_export_annotations,
-            callback=dm.views.export_task_as_dataset
-        )
+        if self._object.data:
+            return self.export_annotations(
+                request=request,
+                pk=pk,
+                db_obj=self._object,
+                export_func=_export_annotations,
+                callback=dm.views.export_task_as_dataset)
+        else:
+            return Response(data="Exporting a task without data is not allowed",
+                status=status.HTTP_400_BAD_REQUEST)
 
 @extend_schema(tags=['jobs'])
 @extend_schema_view(