laravel : sanitize request data before validation

there is a UpdateUserRequest form request that verify fields value against its rules defined in rules mathod .it has rules() and authorize() methods by default. i want prevent verifying and updating empty fields (such as password) .

using sometimes in rules is not useful as html inputs will be present in POST request even if they are empty.

array:6 [▼
 "_method" => "PATCH"
 "_token" => "Cz79rRez2f6MG0tTU17nVwXD0X1lNGH1hA7OORjm"
 "name" => "john"
 "email" => "[email protected]"
 "password" => ""
 "password_confirmation" => ""

]

so i should remove empty keys of POST request before using sometimes in rules.
the question is : Where is the best place to purge Request array?
is there any laravel build in method to manage such situations?

P.S :Solution:
@Bogdon solution is still valid and works, but there is another simple ,nice ,neat solution adopted from here:
just override all() method inside form request

 class RegistrationRequest extends Request
  {

...

public function all()
{
    $attributes = parent::all();

    if(isset($attributes['password']) && empty($attributes['password'])) 
        {
            unset($attributes['password']);
        }
    $this->replace($attributes);

    return parent::all();

}

...

}

Solution

To make this work you'll need to modify the contents of the App\Http\Requests\Request class to allow a way to sanitize the input (class code taken from this Laracasts post):

namespace App\Http\Requests;

use Illuminate\Foundation\Http\FormRequest;

abstract class Request extends FormRequest
{
    /**
     * Validate the input.
     *
     * @param  \Illuminate\Validation\Factory  $factory
     * @return \Illuminate\Validation\Validator
     */
    public function validator($factory)
    {
        return $factory->make(
            $this->sanitizeInput(), $this->container->call([$this, 'rules']), $this->messages()
        );
    }

    /**
     * Sanitize the input.
     *
     * @return array
     */
    protected function sanitizeInput()
    {
        if (method_exists($this, 'sanitize'))
        {
            return $this->container->call([$this, 'sanitize']);
        }

        return $this->all();
    }
}

After that you just need to write add sanitize method in the UpdateUserRequest class that removes the password field from the input when it's empty:

public function sanitize()
{
    if (empty($this->get('password'))) {
        // Get all input
        $input = $this->all();
        // Remove the password field
        unset($input['password']);
        // Replace the input with the modified one
        $this->replace($input);
    }

    return $this->all();
}

Now using the sometimes rule for the password field will work:

public function rules()
{
    return [
        // Other rules go here
        'password' => 'sometimes|required|confirmed'
    ];
}