Solved and now preventing the non-null but empty data_json annotations.

[Kenneth-Funes-TU-Delft]

• Fixed an issue that when the video failed to load and the user would try to annotate, it would create a data_json array which was non null, but empty. This would then become a "valid" annotation from the point of view of progress and task completion. This should now be prevented.
• Progress is now calculated considering empty arrays as not valid, thus allowing to send again the url to annotators.
• Stopped using the selectedAnnotationIndex from redux, and instead created a useState hook for the continuous annotation task because the custom dispatch appears to break the functionality when the FINISHED state is sent.
• Exporting the prolific_study_id into the results json files