Fix branches API specs through praefect

What does this MR do and why?

Fixes gitaly#4400 (closed)

Example failure https://gitlab.com/gitlab-org/gitlab/-/jobs/2796510844

The before block at the start of this test assumes there is always a repository to create a branch on, but some of tests actually create a project without a repo. This silently fails when using gitaly, but praefect cannot route a request without an existing repository and so explodes. We shouldn't be trying to create branches on repositories that don't exist.

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