How to validate an input field if value is not null in Laravel

In case column is nullable

'password' => 'nullable|min:6|confirmed',

@Rejinderi's answer is correct!

In case column is not nullable (This may help other)

'password' => 'sometimes|required|min:6|confirmed',

Result

username = 'admin',
password = null // fail- required

username = 'admin',
password = 123 // fail- min:6

username = 'admin' // pass- validate only exist

This is how I would do it:

//For new or create :
$this->validate($request, [
    'name' => 'required|max:255',
    'email' => 'required|email|max:255|unique:users',
    'password' => 'required|min:6|confirmed',
]);

//For edit or update:
$this->validate($request, [
    'name' => 'required|max:255',
    'email' => 'required|email|max:255|unique:users',
    'password' => 'min:6|confirmed',//just remove required from password rule
]);

Explanation:
This way the value will be validated only when it will be defined (present) in request
If you use nullable, than validator will accept null as value (which i assume is not acceptable)
If you remove password validation from update than this input will not be validated at all, and any value will be accepted (which is again not acceptable);

we can achieve this also using Validator Class's sometimes method as per Documentation

use Illuminate\Support\Facades\Validator;

//... your code ...

$validator = Validator::make($request->all(), [
      'name' => 'required',
      'contact' => 'required',
]);
$validator->sometimes('password', 'min:6|confirmed', function ($input) {
    return (strlen($input->password) > 0);
});
if ($validator->fails()) {
     return redirect()->back()->withErrors($validator)->withInput();
} 

this will validate password attribute rules when password field's length is greater than 0. otherwise will ignore that password attribute's rule.