Unless you find a guarantee of this (I see none), I wouldn't assume it's true. Make your own sentinel function that returns something you know can't point to a valid object on the platforms you support and document that you don't support any others if it's not otherwise inherent.
Alternatively, file an issue to add the guarantee that it's always the alignment and see if the lib team agrees.
The other alternative to being "silly" is to occasionally break in subtle or not-so-subtle ways when the non-guarantee changes.