laravel-admin 字段非空并设有默认值，表单提交时仍然提示Integrity constraint violation

app/Http/Kernel.php中

    protected $middleware = [
        // \App\Http\Middleware\TrustHosts::class,
        \App\Http\Middleware\TrustProxies::class,
        \Fruitcake\Cors\HandleCors::class,
        \App\Http\Middleware\PreventRequestsDuringMaintenance::class,
        \Illuminate\Foundation\Http\Middleware\ValidatePostSize::class,
        \App\Http\Middleware\TrimStrings::class,
//        \Illuminate\Foundation\Http\Middleware\ConvertEmptyStringsToNull::class,
    ];

\Illuminate\Foundation\Http\Middleware\ConvertEmptyStringsToNull::class, 
这行注释掉，作用是把空字符转换成null，mysql被写入null，提示Integrity constraint violation 完整性约束