[Bluetooth][Non-ACR] Fix unhandled exception in GetBondedDevice() (#2514)

[Bluetooth][Non-ACR] Fix unhandled exception in GetBondedDevice() (#2514)

Device is not bonded : System.NullReferenceException: Object reference not set to an instance of an object.
    at System.SpanHelpers.IndexOf(Byte& searchSpace, Byte value, Int32 length)
    at System.String.Ctor(SByte* value)
    at System.Runtime.InteropServices.Marshal.PtrToStringAnsi(IntPtr ptr)
    at Tizen.Network.Bluetooth.BluetoothUtils.ConvertStructToDeviceClass(BluetoothDeviceStruct device)
    at Tizen.Network.Bluetooth.BluetoothAdapterImpl.GetBondedDevice(String address)
    at Tizen.Network.Bluetooth.BluetoothAdapter.GetBondedDevice(String address)

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