Return undefined instead of a null signature

lib/transaction/input/multisigscripthash.js

@@ -43,7 +43,7 @@ MultiSigScriptHashInput.prototype.toObject = function() {
 MultiSigScriptHashInput.prototype._deserializeSignatures = function(signatures) {
   return _.map(signatures, function(signature) {
     if (!signature) {
-      return signature;
+      return undefined;
     }
     return {
       publicKey: new PublicKey(signature.publicKey),
@@ -59,7 +59,7 @@ MultiSigScriptHashInput.prototype._deserializeSignatures = function(signatures)
 MultiSigScriptHashInput.prototype._serializeSignatures = function() {
   return _.map(this.signatures, function(signature) {
     if (!signature) {
-      return signature;
+      return undefined;
     }
     return {
       publicKey: signature.publicKey.toString(),

lib/transaction/unspentoutput.js

@@ -98,7 +98,7 @@ UnspentOutput.prototype.toJSON = function() {
  */
 UnspentOutput.prototype.toObject = function() {
   return {
-    address: this.address.toString(),
+    address: this.address ? this.address.toString() : undefined,
     txid: this.txId,
     vout: this.outputIndex,
     scriptPubKey: this.script.toBuffer().toString('hex'),