Hey Dharshan, I guess you just found a bug.
BUT, oddly enough, I can not replicate it.
- I made a topic first, tried to post it without the subject, it wouldn't let me through.
- I tried to make a reply and removed the subject, but it still would not let me through.
- I modified the reply and the first topic subject but it still would not let me through. It will give out a message saying No subject was filled or something over those lines.