[SIGMON-562] support empty df in decode_states
Context
What was done
- decode_states can handle empty dataframe
TODO
- ? Anything still to be done?
QA
- test added
Review objectives
- ? assessment of code functionality
- ? assessment of code quality
- ? identifying potential refactorings / logic common to other projects
- ? knowledge sharing
- ? getting help to understand a piece of code
Closes SIGMON-642