See the following:
stackoverflow.com/questions/14848326/emp...n-html-editor-or-not
I found that if you put a type a space then press carriage return, the space makes it a "non-empty" paragraph which seems to work. The line break approach should work also, but maybe only from inside a paragraph?
You can also use the <div> icon to put some of your content in a div which will allow you to set a style on the div...
e.g.
margin-bottom:20px
BTW, it is not the editor doing this space removal. If you press Ctrl-U so that you can see the underlying HTML that is being sent to your browser, you will see that all the <p></p> tags are there. It is the browser that is removing the blank paragraphs.