I run into this often, as I edit a lot of text on Ubuntu, and sometimes instead of pressing the CTRL + V key to paste I press the CTRL + C key on the blank line I'm trying to paste on. What this does is very annoying, now my clipboard is overwritten with 'nothing'.
I don't know if it's now a blank character or what, but it's silly, I should still be able to keep what I had previously in the clipboard if I accidentally do a copy on a blank line.
How do I prevent this from happening?
You could try using a Clipboard manager, several of them are listed here: http://www.webupd8.org/2010/08/best-linux-clipboard-manager.html
Not Sure if there is a way to disable that kind of behavior using one though.