Fix upload-file-experiment-trigger setting base route

What does this MR do?

This MR fixes #332282 (closed), where the browser URL was being changes to something erroneous because a VueRouter was given an incomplete base path.

Also, please check out this investigation.

Screenshots (strongly suggested)

How to test?

You'll need to enable the experiment :empty_repo_upload referenced here by running Feature.enable(:empty_repo_upload) in a rails console.

What should I look for?

Notice that before the browser URL's route changes after visiting the page. When fixed, this should not change.

Before (on master)	After (this MR)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.

Related to #332282 (closed)