When wifi ssid is empty, return WifiSsid.NONE

Issue 199610

When WifistateMachine enter Roaming State, WifiInfo
getSSID will return 0x, statusbar show "0x", because
roaming State do not update WifiInfo mWifiSsid.

Cherry pick from I241f8e9c11dc16db26cadde133043745f2f19573

BUG=27403109
TEST=compile and unittests
Signed-off-by: zhangshuxiao <zhangshuxiao@xiaomi.com>

Change-Id: Ieb7429511898a5ed3d44fd15fc1f250c122e802a