It takes a lot longer to figure out if it'll be a problem than to just add the check. And you don't have to ponder whether it's possible for a null to get there, because now it's fine if it does.
Are you talking about extending the API contract to allow for NULL? That is often the path to madness, especially if it requires complicating the signature (return value etc). Better to just assert/crash.
Because no one is expecting it to work if a null is passed. Your total range of behaviours left are crashes, doesn't crash and is silently ok, or doesn't crash and causes something worse (data corruption, you get your product in a CVE, that area).
My proposition is that "it's silently ok" isn't likely enough, which is in line with your position on "don't extend the contract to accept null". So what's left is crash, or something worse.
So if those are your choices, don't waste time justifying that a null can't get there, just add a check to ensure you get the better behaviour. It takes seconds.
If you follow that line of reasoning, you will end up testing almost every pointer before accessing it. The reason is that you are extending your valid state space massively since you aren't able to specify "this subset of 7 trillion distinct states is invalid, if it was the case we would have failed before".
You are requiring yourself to find a valid outcome for an input that doesn't make _any_ sense in the context of what your application is meant to achieve. How is that not a Sysiphean task?
You're not "extending" the valid state space. That null value being passed to that function is already a potential state of your program.
You're actually pruning the valid state space; before, when the null value is passed to the function, there are more operations performed that have uncertain consequences. If you assert-and-fail when you get the null input, you've pruned those states.