yaml: unify 0 stats counter config option terms

yaml: unify 0 stats counter config option terms

When we added feature #5976 (72146b969), we overlook that we also have
a config stats option for the human-readable stats logs to output
0 counters.
Due to not seeing this before, we now have two different setting names
for basically the same thing, but in different logs:
- zero-valued-counters for EVE
- null-values for stats.log

This ensures we use the same terminology, and change the recently added
one to `null-values`, as this one has been around for longer.

Task #6962