Gracefully handle case when repository's root ref does not exist

This was failing regularly with an Error 500 when the API branches endpoint
was used.

Closes #40615

changelogs/unreleased/sh-fix-root-ref-repository.yml

+---
+title: "Gracefully handle case when repository's root ref does not exist"
+merge_request:
+author:
+type: fixed

lib/gitlab/git/repository.rb

@@ -1253,7 +1253,11 @@ module Gitlab
 
       # Gitaly migration: https://gitlab.com/gitlab-org/gitaly/issues/695
       def git_merged_branch_names(branch_names = [])
-        root_sha = find_branch(root_ref).target
+        return [] unless root_ref
+
+        root_sha = find_branch(root_ref)&.target
+
+        return [] unless root_sha
 
         git_arguments =
           %W[branch --merged #{root_sha}

spec/lib/gitlab/git/repository_spec.rb

@@ -1210,6 +1210,16 @@ describe Gitlab::Git::Repository, seed_helper: true do
       end
     end
 
+    context 'when no root ref is available' do
+      it 'returns empty list' do
+        project = create(:project, :empty_repo)
+
+        names = project.repository.merged_branch_names(%w[feature])
+
+        expect(names).to be_empty
+      end
+    end
+
     context 'when no branch names are specified' do
       before do
         repository.create_branch('identical', 'master')