[Backport] Go-get: fix 401 error for unauthenticated requests

What does this MR do and why?

Contributes to #493732 (closed)

Problem

Self-managed instances that restricted password authentication for Git over HTTP(S) started to receive 401 error code for go-get=1 requests from go toolchain.

The reason is a missing return for the case when request doesn't have basic credentials.

It was introduced in !161162 (merged).

Solution

Restore check for missing basic credentials and add a test case.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

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• Refer to the patch release runbook for engineers and maintainers for guidance.
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