Support NULL elements in PostgreSQL arrays

[alexeichhorn]

Problem

dbkit supports:

Vec<String>         // required TEXT[] value with non-null elements
Option<Vec<String>> // nullable TEXT[] value with non-null elements

It does not support PostgreSQL arrays containing nullable elements:

Vec<Option<String>>
Option<Vec<Option<String>>>

Using Vec<Option<String>> in a #[model] currently fails during macro expansion because dbkit::Value only supports Vec<String>. Conversely, reading a valid PostgreSQL value such as ARRAY['one', NULL, 'three'] into Vec<String> fails during decoding.

Expected behavior

• Support Vec<Option<String>> for a present TEXT[] containing nullable elements.
• Support Option<Vec<Option<String>>> so container and element nullability compose.
• Preserve existing Vec<String> and Option<Vec<String>> behavior.
• Cover insertion, updates, selection, and binding where arrays are currently supported.
• Test empty arrays, mixed values/NULLs, all-NULL elements, and a NULL array.

This can start with the existing TEXT[] support; it does not need to generalize every PostgreSQL array element type at the same time.

PostgreSQL array NULL elements