Auto merge of #84842 - blkerby:null_lowercase, r=joshtriplett

Auto merge of #84842 - blkerby:null_lowercase, r=joshtriplett

Replace 'NULL' with 'null'

This replaces occurrences of "NULL" with "null" in docs, comments, and compiler error/lint messages. This is for the sake of consistency, as the lowercase "null" is already the dominant form in Rust. The all-caps NULL looks like the C macro (or SQL keyword), which seems out of place in a Rust context, given that NULL does not exist in the Rust language or standard library (instead having [`ptr::null()`](https://doc.rust-lang.org/stable/std/ptr/fn.null.html)).