Prevent panic on looking at api "git" endpoints for empty repos (#22457) (#22458)

Backport #22457

The API endpoints for "git" can panic if they are called on an empty
repo. We can simply allow empty repos for these endpoints without worry
as they should just work.

Fix #22452

Signed-off-by: Andrew Thornton <art27@cantab.net>

routers/api/v1/api.go

@@ -1042,7 +1042,7 @@ func Routes(ctx gocontext.Context) *web.Route {
 					m.Get("/blobs/{sha}", repo.GetBlob)
 					m.Get("/tags/{sha}", repo.GetAnnotatedTag)
 					m.Get("/notes/{sha}", repo.GetNote)
-				}, context.ReferencesGitRepo(), reqRepoReader(unit.TypeCode))
+				}, context.ReferencesGitRepo(true), reqRepoReader(unit.TypeCode))
 				m.Post("/diffpatch", reqRepoWriter(unit.TypeCode), reqToken(), bind(api.ApplyDiffPatchFileOptions{}), repo.ApplyDiffPatch)
 				m.Group("/contents", func() {
 					m.Get("", repo.GetContentsList)