fix(membership): use 'or ""' to guard against null email/full_name

u.get("key", "") returns None when key exists with null value in MongoDB,
causing Pydantic ValidationError on MemberDetail.email/full_name: str → 500.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/membership_service.py

@@ -124,8 +124,8 @@ async def list_org_members(
             MemberDetail(
                 membership_id=str(doc["_id"]),
                 user_id=doc.get("user_id", ""),
-                email=u.get("email", ""),
-                full_name=u.get("full_name", ""),
+                email=u.get("email") or "",
+                full_name=u.get("full_name") or "",
                 role_in_org=role,
                 created_at=doc.get("created_at"),
             )