Fix AdminInfo#getExecAverage divide by zero

AdminInfo#getExecAverage throws divide by zero exception when
blockExecTime#size is 0 [Delivers #88031650]

ethereumj-core/src/main/java/org/ethereum/manager/AdminInfo.java

@@ -43,6 +43,8 @@ public class AdminInfo {
 
     public Long getExecAvg(){
 
+        if (blockExecTime.size() == 0) return 0L;
+
         long sum = 0;
         for (int i = 0; i < blockExecTime.size(); ++i){
             sum += blockExecTime.get(i);