fix: prefer codec over driver.Valuer for typed-nil in array/composite encoding

[waterWang]

Description

When a typed-nil value in an array or composite field implements driver.Valuer, the array and composite codecs bypass the element type's registered Codec and directly call encodePlanDriverValuer. This is incorrect when the type has a registered Codec that should handle nil values (typically encoding as SQL NULL).

This was introduced in #2567.

Fix

Replace the encodePlanDriverValuer shortcut with PlanEncode, which naturally gives precedence to the Codec's PlanEncode before falling back to driver.Valuer encoding. This ensures that types with both a Codec and driver.Valuer use the Codec's nil handling.

Changes

• pgtype/array_codec.go (text + binary encode paths): Replaced direct encodePlanDriverValuer call with PlanEncode + caching by elemType (same pattern as the non-nil path)
• **pgtype/composite.go\** (text + binary builder): Replaced direct encodePlanDriverValuerwithPlanEncode(with fallback toencodePlanDriverValuer` if nil)

Fixes #2611

[jackc]

Is this just reversing #2567?

When a typed-nil value in an array or composite field implements driver.Valuer, the array and composite codecs bypass the element type's registered Codec and directly call encodePlanDriverValuer. This is incorrect when the type has a registered Codec that should handle nil values (typically encoding as SQL NULL).

As was mentioned in the issue, that is incorrect.

pgx/pgtype/doc.go

Lines 145 to 153 in 6e4e3e7

	Encoding Typed Nils
	pgtype encodes untyped and typed nils (e.g. nil and []byte(nil)) to the SQL NULL value without going through the [Codec]
	system. This means that [Codec]s and other encoding logic do not have to handle nil or *T(nil).
	However, [database/sql] compatibility requires Value to be called on T(nil) when T implements [database/sql/driver.Valuer]. Therefore,
	[database/sql/driver.Valuer] values are only considered NULL when *T(nil) where [database/sql/driver.Valuer] is implemented on T not on *T. See
	https://github.com/golang/go/issues/8415 and
	https://github.com/golang/go/commit/0ce1d79a6a771f7449ec493b993ed2a720917870.