`f.length()` returns 0 instead of throwing error if f does not exist
`f.length()` returns 0 instead of throwing error if f does not exist

Not sure if we should send back a `400` instead, but this change can be
justified given the existing code sends back `None` if `f` is not a
file.

aeons/http4s,ChristopherDavenport/http4s,hvesalai/http4s,rossabaker/http4s,ZizhengTai/http4s,m4dc4p/http4s,aeons/http4s,reactormonk/http4s,aeons/http4s,hvesalai/http4s,m4dc4p/http4s,hvesalai/http4s,ZizhengTai/http4s,reactormonk/http4s,ChristopherDavenport/http4s,ZizhengTai/http4s,reactormonk/http4s,http4s/http4s,m4dc4p/http4s,ChristopherDavenport/http4s,rossabaker/http4s