Fix 'New merge request' button

What does this MR do?

On the 'Merge requests' page of a project, there's an New merge request button in the top navigation on the right. If the list of merge requests is empty, another New merge request button is also displayed in the list area. However, this button leads to a ~bug for members of a fork:

A member of a fork has the permission to create a merge request in the fork, but not in the project itself. However, the button in the empty list links to the New merge request page of the project instead of the fork.

user	button in top navigation	button in empty list
member of the project	<full project URL>/-/merge_requests/new ✅	<full project URL>/-/merge_requests/new ✅
member of a fork but not of the project	<full fork URL>/-/merge_requests/new ✅	<full project URL>/-/merge_requests/new 🔴 404
no member of the project	no button ✅	no button ✅

This MR fixes the ~bug so that the button in the empty list also links to the same page as the button in the top navigation.

🛠 with ❤ at Siemens

/cc @bufferoverflow

Screenshots or Screencasts

Note: see the description of the steps in the How to setup and validate locally chapter below

before	after
New_merge_request_forked_before	New_merge_request_forked_after

How to setup and validate locally

1. Sign in as megan (member of the Newton Bayer/Lab Coat fork of the Commit451/Lab Coat project).
2. Visit 'Merge requests' page of Commit451/Lab Coat: http://localhost:3000/Commit451/lab-coat/-/merge_requests?scope=all&state=opened&search=foo
3. Click on New merge request button in top navigation: ➡ link to http://localhost:3000/megan/lab-coat/-/merge_requests/new ✅
4. Go back to 'Merge requests' page
5. Before MR: Click on New merge request button in empty list: ➡ link to http://localhost:3000/Commit451/lab-coat/-/merge_requests/new 🔴 404
6. After MR: Click on New merge request button in empty list: ➡ link to http://localhost:3000/megan/lab-coat/-/merge_requests/new ✅

Does this MR meet the acceptance criteria?

Conformity

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• 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.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] 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