fix: grouped product special price calculation

[bartoszherba]

Description

Actual behavior

The total price is calculated without including the special price of the first product - looks like the first product in the group is treated as a "normal" priced product.
Expected behavior

The total price is calculated correctly by using special prices associated with product.
Additional bug information

Reproduced with Chromium-based browsers (Chrome, Edge), Safari, and Firefox.

How Has This Been Tested?

change grouped product options and observe if normal and special prices are calculated properly in the total result

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.