Avoid returning NaN when requesting for count metrics per second and …

…no metrics have been recorded

This is mainly to avoid failing execution when trying to deserialize the statistics from dotnet dsl.

jmeter-java-dsl/src/main/java/us/abstracta/jmeter/javadsl/core/stats/CountMetricSummary.java

@@ -24,7 +24,7 @@ public void increment(long inc, long elapsedTimeMillis) {
    * Provides the average count per second for the given metric.
    */
   public double perSecond() {
-    return val / ((double) elapsedTimeMillis / 1000);
+    return elapsedTimeMillis != 0 ? val / ((double) elapsedTimeMillis / 1000) : 0;
   }
 
   /**