https://discussions.apple.com/thread/4016883?sortBy=best
Automator doesn’t have any control actions, so any comparisons you will need to do yourself. This is easy though, you can run a script after your Filter Paragraphs action to quit the workflow if the result is empty - for example, add a Run AppleScript action after the filter/before the new message and paste the following script into it:
on****run {input, parameters}
ifinputis {} then****error-128_— cancel on empty input list_
return****input
end****run