pacakge/iw: don't show a stray ']' if the chain signal attribute list is empty

pacakge/iw: don't show a stray ']' if the chain signal attribute list is empty

Without the patch, the output of 'station dump' command looks weird in
some cases:

root@OpenWrt:/# iw dev wlan0 station dump
Station 00:21:27:cb:57:02 (on wlan0)
        inactive time:  190 ms
        rx bytes:       36652
        rx packets:     149
        tx bytes:       1998
        tx packets:     16
        tx retries:     0
        tx failed:      0
        signal:         -31 ] dBm
        signal avg:     -30 [-32, -36] dBm
        tx bitrate:     5.5 MBit/s
        rx bitrate:     54.0 MBit/s
root@OpenWrt:/#

SVN-Revision: 28879