WIP: Handle non-existent start branch in UserCommitFiles

UserCommitFiles receives requests where the StartBranch is set even for an empty repository. Such requests fail when attempting to resolve the start branch' commit as the branch does not exist. The Ruby implementation of the RPC went around this case by checking whether the repository has no branches. If so, it ignored the start branch. This scenario caused failures in the QA pipeline. It doesn't appear such requests are sent in production though.

This commit fixes the failing pipeline by mirroring the Ruby implementations behavior and ignoring the start branch when the repository has no branches. The Go implementation already handled this for the remote repository. This changes the handling to also take place when the start repository is not set.

Fixes the issues leading disabling the go_user_commit_files feature flag in !2952 (merged) and enables the feature flag again.

QA pipeline: gitlab!50606 (closed)