MINOR: activity: better report nil than ffff in unknown callers

MINOR: activity: better report nil than ffff in unknown callers

For unknown callers we try to get the lowest known address and we
purposely ignore NULL during calculation of the min. But the side
effect is that we also report ffff in the per-DSO address. Better
catch this case and finally accept to report nil. Before it would
report this:

  $ socat - /tmp/sock1 <<< "show profiling memory" |grep nil
        50000          10        9600000           9440|            (nil) [other] unknown(192) [delta=9590560] [pool=http_txn]
        50000          10        9600000           9440|            (nil) DSO:other; delta_calls=49990; delta_bytes=9590560

now it reports this:

  $ socat - /tmp/sock1 <<< "show profiling memory" |grep nil
        50000          11        9600000           9656|            (nil) [other] unknown(192) [delta=9590344] [pool=connection]
        50000          11        9600000           9656|            (nil) DSO:other; delta_calls=49989; delta_bytes=9590344