[SIGMON-530] fix NB_FPA notebook
Context
- Zinur spotted some bugs in the RB_FPA notebook when neither nQPS is dumped nor logs present in NXCALS.
What was done
- add some nullity conditions
TODO
- fix the second error in 8.3
QA
- code was not unit tested, because we can quickly run the notebook to reproduce the issue and make sure the code was actually fixed
- acceptance tests were written
Review objectives
- code functionality
- code quality