[dotnet] Add not-null Response.Value helper (#15243)

* [dotnet] Add not-null `Response.Value` helper

* Use other style

* update method name

* add custom error message

* EnsureValueIsNotNull can return itself

* Improve message

* Improve message

Author: RenderMichael

dotnet/src/webdriver/Response.cs

@@ -20,6 +20,7 @@
 using OpenQA.Selenium.Internal;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Text.Json;
 using System.Text.Json.Serialization;
@@ -215,6 +216,21 @@ public string ToJson()
             return JsonSerializer.Serialize(this);
         }
 
+        /// <summary>
+        /// Throws if <see cref="Value"/> is <see langword="null"/>.
+        /// </summary>
+        /// <exception cref="WebDriverException">If <see cref="Value"/> is <see langword="null"/>.</exception>
+        [MemberNotNull(nameof(Value))]
+        internal Response EnsureValueIsNotNull()
+        {
+            if (Value is null)
+            {
+                throw new WebDriverException("Response from remote end doesn't have $.Value property");
+            }
+
+            return this;
+        }
+
         /// <summary>
         /// Returns the object as a string.
         /// </summary>