[fix] null values should merge properly instead of throwing errors

lib/nconf/provider.js

@@ -539,7 +539,7 @@ Provider.prototype._execute = function (action, syncLength /* [arguments] */) {
       response = store[action].apply(store, args);
 
       // Merge objects if necessary
-      if (action === 'get' && typeof response === 'object' && !Array.isArray(response)) {
+      if (response && action === 'get' && typeof response === 'object' && !Array.isArray(response)) {
         mergeObjs.push(response);
         response = undefined;
       }

test/fixtures/data.js

@@ -6,6 +6,7 @@
  */
  
 exports.data = {
+  isNull: null,
   literal: 'bazz', 
   arr: ['one', 2, true, { value: 'foo' }],
   obj: {