Allow unset topics in posts too.

lib/web3/formatters.js

@@ -159,6 +159,9 @@ var inputPostFormatter = function(post) {
     post.priority = utils.fromDecimal(post.priority);
 
     // fallback
+    if (post.topics == undefined) {
+        post.topics = []
+    }
     if (!utils.isArray(post.topics)) {
         post.topics = [post.topics];
     }