Returns null not empty string if MEID is not available

TelephonyManager#getMeid is defined to return null if MEID is not
available. However, it may return an empty string. Because Modem returns
an empty string if a device doesn't support CDMA. Therefore, change to
return null if MEID is empty.

Test: cts-tradefed run cts -m CtsDevicePolicyManagerTestCases -t 
com.android.cts.devicepolicy.MixedDeviceOwnerTest#testKeyManagement
Bug: 140472124

Change-Id: I822c352854516e898bbe3d7a6710ecb085991b1b