Support for receiving null messages

Just log em for debugging. Also update the error messge thrown when we get a
content message with no supported properties. It may be empty or may just have
an unrecognized field.

// FREEBIE

js/libtextsecure.js

@@ -38444,11 +38444,17 @@ MessageReceiver.prototype.extend({
                 return this.handleSyncMessage(envelope, content.syncMessage);
             } else if (content.dataMessage) {
                 return this.handleDataMessage(envelope, content.dataMessage);
+            } else if (content.nullMessage) {
+                return this.handleNullMessage(envelope, content.nullMessage);
             } else {
-                throw new Error('Got Content message with no dataMessage and no syncMessage');
+                throw new Error('Unsupported content message');
             }
         }.bind(this));
     },
+    handleNullMessage: function(envelope, nullMessage) {
+        var encodedNumber = envelope.source + '.' + envelope.sourceDevice;
+        console.log('null message from', encodedNumber, envelope.timestamp.toNumber());
+    },
     handleSyncMessage: function(envelope, syncMessage) {
         if (envelope.source !== this.number) {
             throw new Error('Received sync message from another number');

libtextsecure/message_receiver.js

@@ -235,11 +235,17 @@ MessageReceiver.prototype.extend({
                 return this.handleSyncMessage(envelope, content.syncMessage);
             } else if (content.dataMessage) {
                 return this.handleDataMessage(envelope, content.dataMessage);
+            } else if (content.nullMessage) {
+                return this.handleNullMessage(envelope, content.nullMessage);
             } else {
-                throw new Error('Got Content message with no dataMessage and no syncMessage');
+                throw new Error('Unsupported content message');
             }
         }.bind(this));
     },
+    handleNullMessage: function(envelope, nullMessage) {
+        var encodedNumber = envelope.source + '.' + envelope.sourceDevice;
+        console.log('null message from', encodedNumber, envelope.timestamp.toNumber());
+    },
     handleSyncMessage: function(envelope, syncMessage) {
         if (envelope.source !== this.number) {
             throw new Error('Received sync message from another number');