feat(bx): add parse_optional_date for nullable occur dates

Empty values map to None so optional detail dates do not default to today.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/utils/bx_codegen.py

@@ -50,6 +50,20 @@ def parse_reimburse_date(value: str | date | datetime | None) -> date:
     return datetime.strptime(text_value, "%Y-%m-%d").date()
 
 
+def parse_optional_date(value: str | date | datetime | None) -> date | None:
+    """可选日期：空 → None；合法 YYYY-MM-DD / date / datetime → date。"""
+    if value is None:
+        return None
+    if isinstance(value, datetime):
+        return value.date()
+    if isinstance(value, date):
+        return value
+    text_value = str(value).strip()
+    if not text_value:
+        return None
+    return datetime.strptime(text_value, "%Y-%m-%d").date()
+
+
 def format_reimburse_title(applicant_name: str, reimburse_date: date, seq: int) -> str:
     return f"{applicant_name}报销单{reimburse_date.strftime('%Y%m%d')}{seq:03d}"