ptr::replace: make calls on ZST null ptr not UB

[RalfJung]

See https://github.com/rust-lang/rust/issues/138351 for context.

We made ptr::read and ptr::write not UB on ZST null pointers. This does the same with ptr::replace. Since we're just adding a branch on a constant, this should come at no runtime cost.

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[RalfJung]

@rust-lang/libs any thoughts on this? Independent of what exactly we decide in https://github.com/rust-lang/rust/issues/138351, this seems like a basically free way to make replace have less UB and be more consistent with read and write.