use the standard method of checking whether metadata is valid

lib/commands/metadata.js

@@ -19,7 +19,7 @@ Data.getMetadataRaw = function (Env, channel /* channelName */, _cb) {
 
     Env.batchMetadata(channel, cb, function (done) {
         Env.computeMetadata(channel, function (err, meta) {
-            if (!err && meta && meta.channel) {
+            if (!err && HK.isMetadataMessage(meta)) {
                 Env.metadata_cache[channel] = meta;
                 // clear metadata after a delay if nobody has joined the channel within 30s
                 Env.checkCache(channel);