in the case of death of one spouse or both spouses the joint return with respect to the decedent may be made only by his executor or administrator; except that in the case of the death of one spouse the joint return may be made by the surviving spouse with respect to both himself and the decedent if no return for the taxable year has been made by the decedent, no executor or administrator has been appointed, and no executor or administrator is appointed before the last day prescribed by law for filing the return of the surviving spouse. If an executor or administrator of the decedent is appointed after the making of the joint return by the surviving spouse, the executor or administrator may disaffirm such joint return by making, within 1 year after the last day prescribed by law for filing the return of the surviving spouse, a separate return for the taxable year of the decedent with respect to which the joint return was made, in which case the return made by the survivor shall constitute his separate return.