Auto-generate check constraint name when name is empty

[dereuromark]

Resolves #1097

Problem

Adding a check constraint without an explicit name generates invalid SQL. On PostgreSQL:

$this->table('inventory')
    ->addColumn('quantity', 'integer')
    ->addCheckConstraint('quantity >= 0')
    ->create();

emits an empty constraint name:

ALTER TABLE "public"."inventory" ADD CONSTRAINT  CHECK (quantity >= 0);

which PostgreSQL rejects with SQLSTATE[42601]: Syntax error at or near "CHECK".

Root cause

AddCheckConstraint::build() defaults a missing name to an empty string ($options['name'] ?? ''), but every adapter only auto-generates a name when it is null:

$constraintName = $checkConstraint->getName();
if ($constraintName === null) { /* generate */ }

Since Cake\Database\Schema\Constraint types the name as a non-nullable string, getName() never returns null. The auto-generation branch was therefore dead code and an unnamed constraint kept its empty name.

This has been latent since the constraint value object switched to the core CheckConstraint (non-nullable name) in 5.0, and became user-visible once the fluent Table::addCheckConstraint() API landed in 5.1, which is the first path that lets a name go unset.

The same dead check exists in the Postgres, MySQL and SQLite adapters. SQL Server already throws BadMethodCallException for check constraints, so it is unaffected.

Fix

Treat an empty string like null in all three adapters so a name is generated:

if ($constraintName === null || $constraintName === '') { /* generate */ }

Tests

• New testAddCheckConstraintAutoGeneratesName() for the Postgres and SQLite adapters covering the empty-name path (SQLite fails without the fix - the empty quoted identifier is a syntax error).
• Tightened the existing MySQL testAddCheckConstraintWithAutoGeneratedName() to assert the exact generated name. It previously passed even with the bug because the database supplied its own fallback name (check_table2_chk_1 on MySQL, CONSTRAINT_1 on MariaDB); it now asserts migrations supplies the name explicitly on both.