OO-5304: Show nothing instead of "null" if no survey responses

src/main/java/org/olat/core/util/StringHelper.java

@@ -295,6 +295,10 @@ public class StringHelper {
 		return s != null? s: "";
 	}
 	
+	public static String toStringOrBlank(Long value) {
+		return value != null? String.valueOf(value): "";
+	}
+	
 	public static int count(String s, char character) {
 		int count = 0;
 		

src/main/java/org/olat/modules/forms/ui/RubricBarChartsController.java

@@ -132,15 +132,15 @@ public abstract class RubricBarChartsController extends FormBasicController {
 	private List<StatisticRow> getStatisticRows(SliderStatistic sliderStatistic) {
 		List<StatisticRow> rows = new ArrayList<>();
 		if (rubric.isNoResponseEnabled()) {
-			rows.add(new StatisticRow(translate("rubric.report.number.no.responses.title"), String.valueOf(sliderStatistic.getNumberOfNoResponses())));
+			rows.add(new StatisticRow(translate("rubric.report.number.no.responses.title"), StringHelper.toStringOrBlank(sliderStatistic.getNumberOfNoResponses())));
 		}
-		rows.add(new StatisticRow(translate("rubric.report.number.responses.title"), String.valueOf(sliderStatistic.getNumberOfResponses())));
+		rows.add(new StatisticRow(translate("rubric.report.number.responses.title"), StringHelper.toStringOrBlank(sliderStatistic.getNumberOfResponses())));
 		rows.add(new StatisticRow(translate("rubric.report.median.title"), EvaluationFormFormatter.formatDouble(sliderStatistic.getMedian())));
 		rows.add(new StatisticRow(translate("rubric.report.variance.title"), EvaluationFormFormatter.formatDouble(sliderStatistic.getVariance())));
 		rows.add(new StatisticRow(translate("rubric.report.sdtdev.title"), EvaluationFormFormatter.formatDouble(sliderStatistic.getStdDev())));
 		return rows;
 	}
-
+	
 	@Override
 	protected void formOK(UserRequest ureq) {
 		//