INSERT in non-strict mode stores '' instead of the column default '0' #552

Reproduction

CREATE TABLE t (id INT NOT NULL, label VARCHAR(10) NOT NULL DEFAULT '0');

SET sql_mode = '';
INSERT INTO t (id) VALUES (1);

SELECT label FROM t;
-- ''   (MySQL: '0')

Reproduced on trunk (3.1.0) with PHP 8.5 and SQLite 3.54.1. In strict mode, the same INSERT stores '0' correctly. WordPress runs in non-strict mode, so plugin tables with a DEFAULT '0' string column can silently get empty values.

Cause

In non-strict mode, the INSERT emulation leaves out omitted columns that have a default, so that SQLite fills them in, and gives the remaining omitted columns their type's implicit default, such as ''. It checks for a default with a truthiness check:

$has_default = $column['COLUMN_DEFAULT'];

PHP treats the string '0' as false, so a '0' default counts as no default. Checking null !== $column['COLUMN_DEFAULT'] would fix it.

Related

Date/time columns with a stored '0' default take the same path and get a zero value. For numeric zero defaults stored by driver 3.1.0 and older (#541, #542), that's the right value by accident. For YEAR string zeros (#545), MySQL expects 2000. A fix changes what both get.