Don't do OCSP stapling only if the staple file is 0 length.

Don't do OCSP stapling only if the staple file is 0 length.

This allows something external (like ocspcheck) to disable the stapling
deliberatly if it can not retreive a valid staple by truncating the
staple file to indicate "do not provide a staple", while the file not
existin will still be treated as a configuration error
ok claudio@ florian@, and prompted by @jsing