Rollup merge of #126152 - RalfJung:size_of_val_raw, r=saethlin

size_of_val_raw: for length 0 this is safe to call

For motivation, see rust-lang/unsafe-code-guidelines#465, specifically around [here](rust-lang/unsafe-code-guidelines#465 (comment)).
Cc `@rust-lang/opsem`

Author: matthiaskrgr