Check if map is initialized prior using it (kube-burner#711)

## Type of change

- [ ] Refactor
- [ ] New feature
- [ ] Bug fix
- [x] Optimization
- [ ] Documentation Update

## Description

Let's check these maps are initialized before using them to prevent
potential issues

## Related Tickets & Documents

- Related Issue kube-burner/kube-burner-ocp#120
- Closes #

Signed-off-by: Raul Sevilla <[email protected]>

pkg/util/metrics/metrics.go

@@ -45,8 +45,12 @@ func ProcessMetricsScraperConfig(scraperConfig ScraperConfig) Scraper {
 	}
 	// Combine users provided metadata with metrics and summary metadata
 	for k, v := range userMetadata {
-		scraperConfig.SummaryMetadata[k] = v
-		scraperConfig.MetricsMetadata[k] = v
+		if scraperConfig.SummaryMetadata != nil {
+			scraperConfig.SummaryMetadata[k] = v
+		}
+		if scraperConfig.MetricsMetadata != nil {
+			scraperConfig.MetricsMetadata[k] = v
+		}
 	}
 	// MetricsEndpoint has preference over the configuration file
 	if scraperConfig.MetricsEndpoint != "" {