fix doc validation issue

.yamato/upm-ci.yml

 test_editors:
-  - version: 2019.4/china_unity/staging
+  - version: 51fe52edbb02
 test_platforms:
   - name: win
     type: Unity::VM

com.unity.uiwidgets/LICENSE.md

-UIWidgets copyright © 2020 Unity Technologies ApS
+UIWidgets copyright © 2021 Unity Technologies ApS
 
 Licensed under the Unity Companion License for Unity-dependent projects--see [Unity Companion License](http://www.unity3d.com/legal/licenses/Unity_Companion_License). 
 

com.unity.uiwidgets/Runtime/services/keyboard.cs

         public readonly char input;
         public readonly TextInputAction? inputAction;
 
-        public RawInputKeyResponse(bool swallow, char input = '\0', TextInputAction? inputAction = null) {
+        public RawInputKeyResponse(bool swallow, int input = 0, TextInputAction? inputAction = null) {
-            this.input = input;
+            this.input = (char) input;
             this.inputAction = inputAction;
         }
 
                 null);
         }
 
-        public static readonly RawInputKeyResponse swallowResponse = new RawInputKeyResponse(true, '\0', null);
+        public static readonly RawInputKeyResponse swallowResponse = new RawInputKeyResponse(true, 0, null);
     }
 
     interface KeyboardDelegate : IDisposable {
 
         void setEditableSizeAndTransform(Dictionary<string, object> args);
-
+        
         void setStyle(Dictionary<string, object> args);
         void setIMEPos(Offset imeGlobalPos);
 
                             _value = _value.deleteSelection(true);
                         }
                     }
-                    else if (currentEvent.character != '\0') {
+                    else if (currentEvent.character != 0) {
                         _value = _value.clearCompose();
                         char ch = currentEvent.character;
                         if (ch == '\r' || ch == 3) {

com.unity.uiwidgets/Runtime/services/keyboard_key.cs

 
         public override void debugFillProperties(DiagnosticPropertiesBuilder properties) {
             base.debugFillProperties(properties);
-            properties.add(new StringProperty("keyId", keyId.ToString().PadLeft(8, '\0'), showName: true));
+            properties.add(new StringProperty("keyId", keyId.ToString().PadLeft(8, (char)0), showName: true));
             properties.add(new StringProperty("keyLabel", keyLabel, showName: true));
             properties.add(new StringProperty("debugName", debugName, showName: true, defaultValue: null));
         }