How to avoid checking for empty value in Haskell?

How to avoid checking for empty value in Haskell?

I'm trying to grasp how Haskell programs can avoid testing for "empty
value". I am struggling to get rid of case expression in this program:
main = do url:outputPath:[] <- getArgs
let maybeUri = parseURI url
case maybeUri of
Just uri -> download uri outputPath
Nothing -> return ()
My very rough understanding is that I should use monad transformer so that
I can use single mappend on Maybe value within IO monad and 'do' syntax
should be able to support it. How can I achieve this?