reactor: fix build on older boost due to spsc_queue::read_available()
reactor: fix build on older boost due to spsc_queue::read_available()

!empty() is a sufficient replacement.

cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,kjniemi/seastar,scylladb/seastar,scylladb/seastar,avikivity/seastar,dreamsxin/seastar,kjniemi/seastar,kjniemi/seastar,cloudius-systems/seastar,avikivity/seastar,avikivity/seastar,syuu1228/seastar,dreamsxin/seastar,dreamsxin/seastar,syuu1228/seastar,cloudius-systems/seastar