Add blank topics appropriately when rejoining

Fixes issue 3810

Change-Id: Ibf44992d80eee00c3a7f190949f4a55a3be93777
Reviewed-on: http://gerrit.dmdirc.com/919
Reviewed-by: Gregory Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>

src/com/dmdirc/ChannelEventHandler.java

@@ -132,7 +132,13 @@ public final class ChannelEventHandler extends EventHandler implements
                     cChannel.getChannelClient(cChannel.getTopicSetter(), true), cChannel.getTopic());
         }
 
-        if (!bIsJoinTopic || !newTopic.getTopic().isEmpty()) {
+        if (!bIsJoinTopic 
+                || (owner.getCurrentTopic() == null && !newTopic.getTopic().isEmpty())
+                || (owner.getCurrentTopic() != null
+                && !newTopic.getTopic().equals(owner.getCurrentTopic().getTopic()))) {
+            // Only add the topic if it's being changed when we're on the
+            // channel (i.e., not a "joinTopic"), or if it's different to the
+            // one we're expecting
             owner.addTopic(newTopic);
         }
     }