Changing string for apn from empty to when no string is provided by SIM Card....

Changing string for apn from empty to  when no string is provided by SIM Card. This fixes the roaming problem with some virtual network providers, since some of them have an empty string (check with getprop gsm.sim.operator.alpha). For sure no provider will write empty in their SIM cards. Data roaming can be disabled after that. Roaming in foreign countrys will still work since it checks the MCC (Mobile Country Code). Thanks to my friend Sewi for helping me finding this. :)

Change-Id: I26ddb849963701acc8a2b1f36fdd18705d46550d