sctp: improve handling of send() calls with no user data`

In particular, don't report EAGAIN on send() calls with no user
data, which might trigger a KASSERT in asyc IO.

Reported by: syzbot+3b4dc5d1d63e9bd01eda@syzkaller.appspotmail.com

(cherry picked from commit bb995f2ef0e75e2e9cc784fc36fc3eb02d3e3113)