return error if cpufreq's are empty

Signed-off-by: Sergei Semenchuk <pdp.eleven11@gmail.com>

sysfs/system_cpu.go

@@ -11,11 +11,13 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build linux
 // +build linux
 
 package sysfs
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 	"strings"
@@ -178,6 +180,10 @@ func (fs FS) SystemCpufreq() ([]SystemCPUCpufreqStats, error) {
 		return nil, err
 	}
 
+	if len(systemCpufreq) == 0 {
+		return nil, fmt.Errorf("could not find any cpufreq files")
+	}
+
 	return systemCpufreq, nil
 }