Importing a skeleton track with no shapes for one of the points leads to import crash

[Maxim Zhiltsov]

Actions before raising this issue

• I searched the existing issues and did not find anything similar.
• I read/searched the docs

Steps to Reproduce

1. Create a task from backup
   task_test_6779-job-overlap-merge_backup_2024_04_17_12_49_41.zip
2. Import annotations
   annotations-cvat-for-video.zip

Expected Behavior

No response

Possible Solution

Can probably be fixed by adding this here:

    def _modify_unmatched_object(self, obj, end_frame):
        if not obj["shapes"]:
            return

Once done, leads to the problem #6779.

Context

Related #6779
Related #7669

Environment

No response