[Bluetooth][Non-ACR] Add NULL check logic for Marshal.PtrToStringAnsi (#1658)

[Bluetooth][Non-ACR] Add NULL check logic for Marshal.PtrToStringAnsi (#1658)

This patchset is to avoid ArgumentNullException error. There are some
cases to come NULL variable from Native API.

BluetoothAdapterImpl.cs: RegisterDiscoveryStateChangedEvent(258) > Discovery state changed callback is called
Unhandled exception.
System.ArgumentNullException: Value cannot be null. (Parameter 'ptr')
at System.Runtime.InteropServices.Marshal.PtrToStringAnsi(IntPtr ptr, Int32 len)
at Tizen.Network.Bluetooth.BluetoothUtils.ConvertStructToDiscoveredDevice(BluetoothDiscoveredDeviceStruct structDevice)
at Tizen.Network.Bluetooth.BluetoothAdapterImpl.<RegisterDiscoveryStateChangedEvent>b__45_0(Int32 result, BluetoothDeviceDiscoveryState state, IntPtr deviceInfo, IntPtr user

Signed-off-by: DoHyun Pyun <dh79.pyun@samsung.com>