Adds error handling when empty PAT is provided

What does this MR do and why?

Recently, due to a misnamed environment variable, the value being returned when accessing the environment variable was just an empty string so the test continued along and filled the field with an empty string causing an error down the line. This change is just to help surface the issue that an empty string was provided.

Related to #406273 (closed)

How to set up and validate locally

Run qa/specs/features/ee/browser_ui/4_verify/pipeline_for_project_mirror_github_spec.rb against Staging without providing a QA_GITHUB_ACCESS_TOKEN environment variable.

export GITLAB_USERNAME=<1Password>
export GITLAB_PASSWORD=<1Password>
export GITLAB_QA_ACCESS_TOKEN=<1Password>
export GITLAB_ADMIN_USERNAME=<1Password>
export GITLAB_ADMIN_PASSWORD=<1Password>
export GITLAB_ADMIN_QA_ACCESS_TOKEN=<1Password>
export GITLAB_QA_USER_AGENT=<1Password>

bundle exec bin/qa Test::Instance::All https://staging.gitlab.com -- qa/specs/features/ee/browser_ui/4_verify/pipeline_for_project_mirror_github_spec.rb

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.