Fix null Exception at _$BluetoothDeviceFromJson and BluetoothManager.state.

[ttyniwa]

This PR solves andrey-ushakov/esc_pos_bluetooth#91

error message:

E/flutter ( 9214): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: type 'Null' is not a subtype of type 'bool' in type cast
E/flutter ( 9214): #0      _$BluetoothDeviceFromJson
package:flutter_bluetooth_basic/src/bluetooth_device.g.dart:14
E/flutter ( 9214): #1      new BluetoothDevice.fromJson
package:flutter_bluetooth_basic/src/bluetooth_device.dart:15
...

[jobypthomas]

@andrey-ushakov Would you please allow this change in?

[andrey-ushakov]

Awesome! Thank you for your contribution!
As I don't have a pos printer currently, it becomes hard to test and maintain the package.
Your PR are always welcome!! :)

[jobypthomas]

@andrey-ushakov when will it be released in pub.dev?