Select Consolidate Blank Lines in the Extra menu to replace all blocks of consecutive totally blank lines with a single blank line. Only lines that have no characters at all are consolidated. Lines that consist solely of whitespace are ignored.
If you want to consolidate lines that have whitespace too, first use Extra|Trim Trailing Whitespace to remove the whitespace, and then use Consolidate Blank Lines.
Extra menu
Extra|Delete Blank Lines
Extra|Trim Trailing Whitespace