Fix 500 error for Tags API when repository is missing

What does this MR do and why?

Problem

Requests to tags API when the repository is missing return "Internal server error".

Solution

Return 404 page if the repository is missing.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a project without repository

   FactoryBot.create(:project, path: 'empty-repository-project', namespace: Namespace.first)

2. Visit http://127.0.0.1:3000/api/v4/projects/root%2Fempty-repository-project/repository/tags
3. You should see 404 error instead of 500 error

MR acceptance checklist

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