fix(phpstan): ajax - improve null checking and function existence verification

Changes include:

1) In `removePos.php`, removed the usage of null safe method call, as it was not needed.
2) In `getOrderControl.php`, added a null guard before trying to get view from the OrderProcess and
use it. If the method does not exist, null will be assigned to `$View`.
3) Removed unnecessary fallback to get the first step in `getStep.php`. The current step is enough.
4) In `savePayment.php` added a check for the existence of 'savePayment' method before invoking it
to avoid possible errors.
5) The fallback to get the first step was also removed in `reload.php`.
6) Removed an unnecessary if condition in `send.php` and directly assigned current step's name to
`$current`.
7) In `setQuantity.php`, added a verification to check the existence of 'setQuantity' method before
calling it.

Related: #172