解决方法：
既然是中间件作祟，肯定在内核中有注册。找到内核注释该中间件就好。
app/Http/Kernel.php

class Kernel extends HttpKernel
{
    /**
     * The application's global HTTP middleware stack.
     *
     * These middleware are run during every request to your application.
     *
     * @var array
     */
    protected $middleware = [
        \App\Http\Middleware\CheckForMaintenanceMode::class,
        \Illuminate\Foundation\Http\Middleware\ValidatePostSize::class,
        \App\Http\Middleware\TrimStrings::class,
        //\Illuminate\Foundation\Http\Middleware\ConvertEmptyStringsToNull::class,//会将''转化为null
        \App\Http\Middleware\TrustProxies::class,
        \Illuminate\Session\Middleware\StartSession::class,
    ];