Prometheus: Fix resource call panic (#50216) (#50221)

(cherry picked from commit 4aa5e7e69e)

Co-authored-by: Todd Treece <360020+toddtreece@users.noreply.github.com>

pkg/tsdb/prometheus/resource/resource.go

@@ -58,7 +58,11 @@ func (r *Resource) fetch(ctx context.Context, client *client.Client, req *backen
 
 	resp, err := client.QueryResource(ctx, req.Method, u.Path, u.Query())
 	if err != nil {
-		return resp.StatusCode, nil, err
+		statusCode := 500
+		if resp != nil {
+			statusCode = resp.StatusCode
+		}
+		return statusCode, nil, err
 	}
 
 	defer resp.Body.Close() //nolint (we don't care about the error being returned by resp.Body.Close())