Resolve "Follow-up from "Set project path on localStorage during onboarding""

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14117 we added the user onboarding MVC. During the onboarding process, users will be guided through the creation of a project. The newly created project will be stored on the localStorage for further usage.

This MR is a follow-up issue which adds a sanity check and returns empty string for the project path in case the relevant input cannot be found on the page (which should btw never be the case).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #12405 (closed)