fix: Index out of range in metrics_writer

2023-08-24T18:08:21.796778057Z 2023/08/24 18:08:21 http: panic serving ip:port: runtime error: index out of range [0] with length 0
2023-08-24T18:08:21.796814387Z goroutine 589687 [running]:
2023-08-24T18:08:21.796823334Z net/http.(*conn).serve.func1()
2023-08-24T18:08:21.796830520Z  /usr/local/go/src/net/http/server.go:1854 +0xbf
2023-08-24T18:08:21.796838117Z panic({0x19c2060, 0xc0043060a8})
2023-08-24T18:08:21.796844764Z  /usr/local/go/src/runtime/panic.go:890 +0x263
2023-08-24T18:08:21.796852064Z k8s.io/kube-state-metrics/v2/pkg/metrics_store.SanitizeHeaders(...)
2023-08-24T18:08:21.796858974Z  /root/go/src/k8s.io/kube-state-metrics/pkg/metrics_store/metrics_writer.go:94
2023-08-24T18:08:21.796866007Z k8s.io/kube-state-metrics/v2/pkg/metricshandler.(*MetricsHandler).ServeHTTP(0xc000221490, {0x1d425a0, 0xc005872700}, 0xc003f9d300)
2023-08-24T18:08:21.796873054Z  /root/go/src/k8s.io/kube-state-metrics/pkg/metricshandler/metrics_handler.go:211 +0x8fe
...

Signed-off-by: Manuel Rüger <manuel@rueg.eu>

pkg/metrics_store/metrics_writer.go

@@ -91,11 +91,13 @@ func (m MetricsWriter) WriteAll(w io.Writer) error {
 func SanitizeHeaders(writers MetricsWriterList) MetricsWriterList {
 	var lastHeader string
 	for _, writer := range writers {
-		for i, header := range writer.stores[0].headers {
-			if header == lastHeader {
-				writer.stores[0].headers[i] = ""
-			} else {
-				lastHeader = header
+		if len(writer.stores) > 0 {
+			for i, header := range writer.stores[0].headers {
+				if header == lastHeader {
+					writer.stores[0].headers[i] = ""
+				} else {
+					lastHeader = header
+				}
 			}
 		}
 	}