修复读取 CCCD 报错的问题，写入时更新特征值 (#50)

* 修复 CCCD 无法读取的问题

* 优化代码

* 调整部分必需参数为可空参数

* 调整接口

* 写入时更新特征值

* 调整接口

* 适配新接口

* 调整依赖项

bluetooth_low_energy_platform_interface/lib/src/gatt_characteristic.dart

@@ -19,7 +19,7 @@ abstract class GattCharacteristic extends GattAttribute {
   factory GattCharacteristic({
     required UUID uuid,
     required List<GattCharacteristicProperty> properties,
-    required Uint8List value,
+    Uint8List? value,
     required List<GattDescriptor> descriptors,
   }) =>
       MyGattCharacteristic(

bluetooth_low_energy_platform_interface/lib/src/gatt_descriptor.dart

@@ -9,7 +9,7 @@ abstract class GattDescriptor extends GattAttribute {
   /// Constructs a [GattDescriptor].
   factory GattDescriptor({
     required UUID uuid,
-    required Uint8List value,
+    Uint8List? value,
   }) =>
       MyGattDescriptor(
         uuid: uuid,

bluetooth_low_energy_platform_interface/lib/src/my_gatt_characteristic.dart

@@ -7,9 +7,9 @@ import 'my_gatt_descriptor.dart';
 
 class MyGattCharacteristic extends MyGattAttribute
     implements GattCharacteristic {
+  Uint8List _value;
   @override
   final List<GattCharacteristicProperty> properties;
-  Uint8List? _value;
   @override
   final List<MyGattDescriptor> descriptors;
 
@@ -18,9 +18,9 @@ class MyGattCharacteristic extends MyGattAttribute
     required this.properties,
     Uint8List? value,
     required this.descriptors,
-  }) : _value = value?.trimGATT();
+  }) : _value = value?.trimGATT() ?? Uint8List(0);
 
-  Uint8List get value => _value ?? Uint8List.fromList([]);
+  Uint8List get value => _value;
   set value(Uint8List value) {
     _value = value.trimGATT();
   }

bluetooth_low_energy_platform_interface/lib/src/my_gatt_descriptor.dart

@@ -4,14 +4,14 @@ import 'gatt_descriptor.dart';
 import 'my_gatt_attribute.dart';
 
 class MyGattDescriptor extends MyGattAttribute implements GattDescriptor {
-  Uint8List? _value;
+  Uint8List _value;
 
   MyGattDescriptor({
     required super.uuid,
     Uint8List? value,
-  }) : _value = value?.trimGATT();
+  }) : _value = value?.trimGATT() ?? Uint8List(0);
 
-  Uint8List get value => _value ?? Uint8List.fromList([]);
+  Uint8List get value => _value;
   set value(Uint8List value) {
     _value = value.trimGATT();
   }