Fix crash when applying empty diff to an empty masternode list (#116)

* Fix crash when applying empty diff to an empty masternode list

Author: antouhou

lib/deterministicmnlist/SimplifiedMNList.js

@@ -53,7 +53,7 @@ SimplifiedMNList.prototype.applyDiff = function applyDiff(simplifiedMNListDiff)
   this.deleteMNs(diff.deletedMNs);
   this.addOrUpdateMNs(diff.mnList);
 
-  this.lastDiffMerkleRootMNList = diff.merkleRootMNList;
+  this.lastDiffMerkleRootMNList = diff.merkleRootMNList || constants.NULL_HASH;
   this.merkleRootMNList = this.calculateMerkleRoot();
 
   if (!this.verify()) {