I echo mercman's message of sympathy. It's a bit early for you to be burdened with such correspondence from the bank.
In general, assets that were jointly owned pass directly to the survivor, and are not part of the estate. I think there might be exceptions, and one of our legal experts might happen by and tell you more.
While you say that there is no estate to speak of, it is possible that you are overlooking some things, such as saleable personal effects. Was there a car in his name, or a set of golf clubs, stuff like that?
It's not unreasonable for the bank to want sight of the will. After all, they have an interest in the estate. They are at least entitled to know who the executor is to be, so that they can lodge a claim.
It is, however, quite unreasonable for them to chase you in your personal capacity for a debt of the estate as they do not have any basis for holding you responsible. If you are the executrix, then it is reasonable for them to take things up with you in due course. It is important that they (and you) treat your legal capacity as executrix as separate from your personal capacity.
It is quite improper to link any problem they have in pursuing a liability of your husband's estate with your personal credit rating. In my opinion, that is so egregious that you should make a formal complaint to the bank about that, and seek to escalate it to the Financial Services Ombudsman.