Textual references to "null" should be unambiguous

[workingjubilee]

There are four different kinds of "null" in PGX:

The SQL Null type. The pointer to NULL (0x0 on most machines, but sometimes also usize::MAX). The null byte aka \0. is_null boolean markers. These all have wildly different implications. PGX should not, in source or documentation, use the term "null" unqualified. Anywhere. Ever.

[workingjubilee]

Postgres also has the concept of a "nullbitmap" and indices into it, which work like the is_null: [bool] periodically witnessed elsewhere, except bitpacked.

• Support for one case of this was added in https://github.com/tcdi/pgx/pull/662

[workingjubilee]

Also see:

• https://github.com/tcdi/pgx/issues/792