finish ConversationActivity on delete of empty thread

Fixes #3086
Closes #3124

// FREEBIE

src/org/thoughtcrime/securesms/ConversationActivity.java

@@ -504,11 +504,11 @@ public class ConversationActivity extends PassphraseRequiredActionBarActivity
       public void onClick(DialogInterface dialog, int which) {
         if (threadId > 0) {
           DatabaseFactory.getThreadDatabase(ConversationActivity.this).deleteConversation(threadId);
+        }
         composeText.getText().clear();
         threadId = -1;
         finish();
       }
-      }
     });
 
     builder.setNegativeButton(R.string.no, null);