Fix data call issue for null APN

When data call is setup on null apn, data call is deactivated
immediately after being set up as Telephony is treating null APN
as provisioning APN. This is because the xml entry for
provisioning APN is empty which is matching empty APN name.

Add isEmpty check for provisioning APN xml entry.

Bug: 22305124
Change-Id: Ifbb3aaaf18926b9209aa05f8346da839178dc803