Fixed some NPS in Filter Toolchain for invalid derived metrics

src/de/tud/kom/p2psim/impl/analyzer/metric/filter/AbstractFilter.java

@@ -206,7 +206,9 @@ public abstract class AbstractFilter<M extends MetricValue<?>> implements
 			if (isOverallMetric) {
 				M overall = getDerivedMetricValueFor(DerivedMetric.this,
 						incoming, null);
-				setOverallMetric(overall);
+				if (overall != null) {
+					setOverallMetric(overall);
+				}
 			} else {
 				for (Host host : hosts) {
 					M perHost = getDerivedMetricValueFor(DerivedMetric.this,

src/de/tud/kom/p2psim/impl/analyzer/metric/filter/StatisticsFilter.java

@@ -83,14 +83,17 @@ public abstract class StatisticsFilter extends
 			Metric<?> derivedMetric, List<Metric<?>> inputs, Host host) {
 		assert inputs.size() == 1;
 		assert host == null;
-		Metric<?> input = inputs.get(0);
+		Metric<? extends MetricValue<?>> input = inputs.get(0);
 		if (input.isOverallMetric()) {
 			throw new AssertionError(
 					"Only available for per-host input metrics.");
 		}
-		LinkedList<MetricValue> mvs = new LinkedList<MetricValue>(
-				input.getAllPerHostMetrics());
-		return new StatisticsMetricValue(mvs);
+		List<? extends MetricValue<?>> values = input.getAllPerHostMetrics();
+		if (values == null || values.isEmpty()) {
+			return null;
+		}
+		LinkedList<MetricValue> mvs = new LinkedList<MetricValue>(values);
+		return new StatisticsMetricValue(mvs);	
 	}
 
 	/**