fix: yaml.safe_load returns none for empty yaml files, propagating as none to callers expecting dict

[kumburovicbranko682-boop]

Description

yaml.safe_load() returns None when given an empty YAML file (or one with only comments). load_config returns this None directly, and get_eval_group passes it through unchanged. Any caller doing config["key"] will get TypeError: 'NoneType' object is not subscriptable. This is a silent misconfiguration bug — an empty or mis-rotated config file produces no useful error message, just a cryptic downstream crash.

Severity: high
File: nemo_skills/evaluation/utils.py

Expected Behavior

The code should handle this case properly to avoid unexpected errors or degraded quality.