Let's say file.txt contains empty lines.

Hello

World

How

are

you

today?

The following sed command will remove the empty lines. The regular expression looks for whitespace (\s*) from the beginning (^) to the end ($) of each line, and if matched, deletes (d) the line.

~]# cat file.txt | sed '/^\s*$/d'
Hello
World
How
are
you
today?