linkapps: output nothing if no apps exist

The standard message can be somewhat confusing (#38695) in that it
outputs “finished linking” language regardless of whether an app is
actually present/linked or not.

This solution just stops it saying anything if there’s no app in the
directory. It needs a little tidying in the output here, but it’s a
discussion point.

Closes Homebrew/homebrew#38728.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>