This is a spin-off out of another problem here for a proposal of improvement.
It could actually be beneficial for built-in accumulate functions to ignore NULLs.
This would be similar behavior of other languages such:
- SQL (e.g.: MySQL, Oracle, ..)
Currently it is up to the rule author to put a guarding condition in the first accumulate <source pattern> to manually ignore null sources to avoid the built-in accumulate functions to break trying to accumulate on NULLs.