[sitecore-jss-nextjs] Fix basePath preservation when doing redirects in redirects-middleware

[yavorsk]

• This PR follows the Contribution Guide
• Changelog updated
• Upgrade guide entry added

Description / Motivation

This PR is a follow-up to #2178 and addresses the following issue:

• when assigning basePath property of NextUrl, if basePath is empty string it gets assigned as '/' instead of ''; this causes redirects to not work properly when basePath is not configured;

Testing Details

• Unit Test Added
• Manual Test/Other (Please elaborate) - test redirects, with and without basePath, in both client side and server side navigation;

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)

[illiakovalenko]

👍