Rollup merge of #52502 - RalfJung:rotate, r=scottmcm

fix unsafety: don't call ptr_rotate for ZST

`rotate::ptr_rotate` has a comment saying
```
/// # Safety
///
/// The specified range must be valid for reading and writing.
/// The type `T` must have non-zero size.
```
So we better make sure we don't call it on ZST...

Cc @scottmcm (author of https://github.com/rust-lang/rust/pull/41670)