fix: hidden() now strips columns whose value is NULL

[mychidarko]

Fixes #42.

Problem

hidden() strips columns with isset($result[$item]). isset() is false for keys whose value is NULL, so any hidden column that happens to be NULL on that row is left in the result. Typical victims: reset_token, remember_token, deleted_at, email_verified_at — exactly the columns people hide.

db()->select('users')->hidden('reset_token')->first();
// → ['id' => 1, 'email' => '…', 'reset_token' => null]   ← still there

Same isset() in both the first() path (Core.php L623) and the all() path (L730).

Fix

isset($result[$item]) → array_key_exists($item, $result) in both places. No other behaviour changes; the dotted relation.column branch is untouched.

Tests

New test hidden removes fields whose value is null in tests/db.test.php (sqlite, reset_token TEXT NULL), covering first() and all(). Fails on v5.x, passes with the change.

alchemy test: 32 passed (78 assertions). alchemy lint: clean.