Not a regular expression, but Notepad++ 6.3.2 has a number of ways of removing blank lines without using a regular expression.
Menu => Edit => Line operations => Remove empty lines
Menu => Edit => Line operations => Remove empty lines (containing blank characters)
Menu => TextFx => TextFx Edit => Delete blank lines
Menu => TextFx => TextFx Edit => Delete surplus blank lines
The two TextFx methods only delete empty lines, but they can be preceded with either of:
Menu => Edit => Blank operations => Trim trailing spaces
Menu => TextFx => vTextFx Edit** => Trim trailing spaces