Fix #7333 Unable to set negative custom option fixed price in admin view.

[dverkade]

Fixes the ability to set a negative custom option price in the backend.

Description

In Magento 1 it was possible to set a negative price change to a custom option. This PR fixes issue #7333 and makes it possible to set a negative price in the backend of Magento. This PR fixes the backend so that the negative price change can actually be entered in the Magento admin. There is a separate PR for the frontend changes which is PR #14975

There also seem to be two other PR's for this issue which are stale and are not moving along:
#13393
#14342

Fixed Issues

1. Unable to set negative custom option fixed price in admin view. #7333: Unable to set negative custom option fixed price in admin view.

Manual testing scenarios

1. Create a new simple product in the backend and try to create new custom options where the price change has a negative value (for instance, "-10").

[dverkade]

Let me check the failing tests.

[VladimirZaets]

Hi, @dverkade, thank you for collaboration. If you need some help I will glad to help you.