Fix crash of 'analyze attack' with rulesets without ATT&CK tags

[SkxOverKill]

If no rule carries an attack tag, calculate_attack_scores() returns an empty dict. The ATT&CK Navigator layer generation then evaluated max(scores.values()) on the empty dict, which raised 'ValueError: max() iterable argument is empty' and aborted the command with a raw traceback and exit code 1, without writing the output file.

Additionally, an explicitly set --max-score 0 was ignored because the previous expression checked truthiness ('max_score or ...') instead of None.

Resolve the maximum to the explicit --max-score if given, otherwise to the highest score present, falling back to 0 for empty rulesets.