[dotnet] Add not-null Response.Value helper

Author: RenderMichael

dotnet/src/webdriver/Response.cs

@@ -215,6 +215,16 @@ public string ToJson()
             return JsonSerializer.Serialize(this);
         }
 
+        /// <summary>
+        /// Gets the <see cref="Value"/> of this response, or throws if it is <see langword="null"/>.
+        /// </summary>
+        /// <returns>The <see cref="Value"/> instance.</returns>
+        /// <exception cref="WebDriverException">If <see cref="Value"/> is <see langword="null"/>.</exception>
+        internal object GetNotNullValue()
+        {
+            return Value ?? throw new WebDriverException("Expected not-null response");
+        }
+
         /// <summary>
         /// Returns the object as a string.
         /// </summary>