remove implicit empty string to null conversion in sanitize()

[techmahedy]

Previously, sanitize() silently converted '' to null for all models, causing unexpected NULL values in the database and potential NOT NULL constraint violations when developers explicitly passed empty strings.

The base implementation now only trims whitespace:

protected function sanitize($value)
{
    if (is_string($value)) {
        $value = trim($value);
    }
    return $value;
}

Developers who need their own required behavior can opt-in by overriding in their model: