Remove single line breaks, keep “empty” lines

Question

Say I have text like the following text selected with the cursor:

This is a test. 
This 
is a test.

This is a test. 
This is a 
test.

I wo

Answer 1

#SingleInstance force

#v::
    Send ^c
    ClipWait
    ClipSaved = %clipboard%

    Loop
    {
        StringReplace, ClipSaved, ClipSaved, `r`n`r`n, `r`n, UseErrorLevel
        if ErrorLevel = 0  ; No more replacements needed.
            break
    }
    Clipboard := ClipSaved
    return