libmm-glib,location-3gpp: don't ignore location updates when MNC is 0
E.g. China Mobile (MCC 460, MNC 0).
$ mmcli -m toby --location-get
/org/freedesktop/ModemManager1/Modem/0
-------------------------
3GPP location | Mobile country code: '460'
| Mobile network code: '0'
| Location area code: '6188'
| Cell ID: '40955'
-------------------------
GPS NMEA traces | Not available
-------------------------
Raw GPS | Not available
-------------------------
CDMA BS | Not available