don't build preemptive conflux circuits if no predicted ports

Conflux circuit building was ignoring the "predicted ports" feature, which aims to make Tor stop building circuits if there have been no user requests lately. This bug led to every idle Tor on the network building and discarding circuits every 30 seconds, which added overall load to the network, used bandwidth and battery from clients that weren't actively using their Tor, and kept sockets open on guards which added connection padding essentially forever.

Bug went in on commit 39c2927d when we added preemptive conflux circuit pools.

Fixes bug 40981; bugfix on 0.4.8.1-alpha.