At the end of the day, the ability, considering that it cannot work on sentient or otherwise soul-possessing life forms and requires continuous physical contact is useless for most applications. It basically means that, by skewing it's answer towards something, it can replace an enchanted object, part-by-part until the entire object is replaced yet the enchantment remains. Of course, that is a slow and time-consuming process, assuming it is even able to replace parts of the enchanted object while still maintaining physical contact with it. Furthermore, that's assuming the enchantment would even need the ability to transfer it, rather than there being an inbuilt way just to switch the vessel, or there being an easier magical way.
Basically, you start with something enchanted. Say a muramasa blade.
The Ship of Theseus is the philosophical question that if you progressively replaced a ship, part by part over time, would it still be the same ship in the end?
By answering it via Null's ability as "Yes", you could then say replace some of the blade's material with something else, slightly altering it's form. The properties would remain the same as it is considered to be the same weapon.
You repeat that process.
Over and over again.
Until the entire weapon has been replaced with a machinegun.
As the current answer to the Ship of Theseus is "Yes", the machinegun and the original cursed katana are technically the same thing. That is, the bloodlust curse still applies to the browning machinegun. Thus the machinegun is supernaturally empowered but demands blood when unsheathed. Except machineguns are never unsheathed as they have no sheaths. Thus it has effectively removed the limitation whilst keeping the benefit.
The main usage now, seeing as the weapon has already been changed to a more useful form, is that repairing it and whatnot using a similar application of the ability means that the weapon is still considered the same, regardless of how many repairs it's undergone.
I interpreted negligible as having no cost and not necessarily requiring a magic roll. I thus limited the ability as much I could so that it would be unlikely to be useful for anything else so that it may qualify for that designation.
In that case, what is the minimum magic requirement to use that ability reliably enough to perform regular maintenance?