Sorry, I need to revise slightly my previous answer. If you follow that solution you will finish with 2 lines combined. I’ve actually considered a slightly better answer.
Find: (\R)\R+
Replace: \1
What this does is find the first CR/LF (you may need to replace \R with the correct CR/LF character your file has). Then there has to be 1 or more additional CR/LF characters. We only keep the first CR/LF character, removing all others. This will cater for situation of multiple empty lines together, as \R+ greedily consumes as many CR/LF characters as possible.
I have to ask the question though, is the line truly empty or does it have 1 or more spaces in it? Spaces will require a different expression. If spaces then:
Find: (\R)\h*\R+
Replace: \1
The caveat here is that \h refers to both a horizontal space, a tab and a line feed. Depending on the character set used a \h might also refer to part of your CR/LF. If so then replacing \h with \s might fix it.
Terry