fix: step1 空章节过滤 + step3 rule_signature None-safe - Closes #21

- step1 _quick_validate 添加 _has_section_content() 过滤空内容章节
  (如仅含"无"字的图片章节)，避免误报低覆盖率警告
- step3 rule_signature 使用 `or {}` 防御 trigger=None 场景
  修复 QE 报告的 step3 AttributeError

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

skills/ir_generation_skill/step3_merge_and_audit.py

@@ -111,8 +111,8 @@ def load_path_enumeration() -> dict:
 def rule_signature(rule: dict) -> str:
     """Generate a dedup signature from path + trigger + actions."""
     path = rule.get("path", [])
-    trigger = rule.get("trigger", {})
-    actions = rule.get("actions", [])
+    trigger = rule.get("trigger") or {}
+    actions = rule.get("actions") or []
 
     conditions = sorted(
         trigger.get("conditions", []), key=lambda c: c.get("signal", "")