Add security check in handle_timeserie

[drscholly]

Checklist

• I have read the Contribution Guide ;
• I have read the Contributor License Agreement (CLA) and understand that the submission of this PR will constitute an electronic signature of my agreement of the terms and conditions of DataGalaxy's CLA ;
• There is an approved issue describing the change when contributing a new feature ;
• I have added tests that prove my fix is effective or that my feature works ;
• I have added necessary documentation (if appropriate) ;

Short description of what this resolves:

This is done to avoid crashing when the object has a timeserie attribute without value.