The point is that the insurance company can claim this but it wouldn't stand up in court.
They would have to demonstrate that it an unbroken door proves negligence on your part - which would mean demonstrating that it is impossible for a thief to get past the lock otherwise.
No, they can simply deny your claim and then the burden of proof is on you to prove that you locked your door and the thief gained entry by picking the lock. Remember, in any dispute the person who wants the money is the one who has to prove their claims.