Properly handle and return empty string for dangling commits in GetBranchName (#11587)

routers/repo/commit.go

@@ -309,6 +309,7 @@ func Diff(ctx *context.Context) {
 	ctx.Data["BranchName"], err = commit.GetBranchName()
 	if err != nil {
 		ctx.ServerError("commit.GetBranchName", err)
+		return
 	}
 	ctx.HTML(200, tplCommitPage)
 }