“Successor of the decedent” means:
(a) If the decedent died leaving a will, the sole beneficiary or all of the beneficiaries who succeeded to a particular item of property of the decedent under the decedent’s will. For the purposes of this part, a trust is a beneficiary under the decedent’s will if the trust succeeds to the particular item of property under the decedent’s will.
(b) If the decedent died without a will, the sole person or all of the persons who succeeded to the particular item of property of the decedent under Sections 6401 and 6402 or, if the law of a sister state or foreign nation governs succession to the particular item of property, under the law of the sister state or foreign nation.