Respect an explicit null modifier when changing a column

[JonasPardon]

On wip/1.3, changing a column with ->default(null) keeps the column's old default instead of removing it:

// earlier migration
$table->string('payment_method', 20)->default('niet betaald');

// later migration: Laravel 9 removed the default, wip/1.3 keeps 'niet betaald'
$table->string('payment_method', 20)->nullable()->default(null)->change();

The ->change() shims copy a modifier from the existing column when the migration didn't set it, and they check that with isset(). isset() is false for null, so an explicit ->default(null) (or ->comment(null), ...) is treated as "not set". This affects fresh installs and any new migration. It came up in wintercms/winter#1366, where a fresh database kept 'niet betaald' as the default.

• Check array_key_exists() on the column's attributes instead, in MySqlBasedGrammar, PostgresGrammar, SqlServerGrammar and BlueprintState (SQLite). Laravel only adds the key when the modifier is called, so unset modifiers are still copied from the existing column.
• Add a test per grammar. All four fail without the change. The SQLite one runs against a real in-memory database. I also checked it on a real MySQL 9.6 database: the default goes from 'niet betaald' to NULL.

🤖 Generated with Claude Code

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