Bluetooth: Send default value for CNUM.

Use case:
1. Connect BT headset.
2. Make MO call.
3. Check for AT+COPS response and SCO establishment.

Failure:
No response sent for AT+COPS.

Root cause:
Telephony is returning null as operator name.

Fix:
 - When null value is returned for operator's
   name and subscriber number from telephony
   APIs, add default values for those.

Change-Id: I3d675e786bece1a18b651d5dba1207cf6603121d