[dotnet] Increment WebDriver towards nullability (#15228)

* [dotnet] Increment `WebDriver` towards nullability

* minimize some diffs with error throwing

* further minimize some diffs with error throwing

* Annotate more of `WebDriver`, leaving only the commands where we need to read the spec for

* fix nullability of adjacent types

* remove changes to Execute

* remove more

* revert final change about Execute

* Executing null command name throws, document this

* Use new `EnsureValueIsNotNull` helper

* cleanup

* `Execute` cannot return `null`

Author: RenderMichael

dotnet/src/webdriver/Command.cs

@@ -55,11 +55,12 @@ public Command(string name, string jsonParameters)
         /// <param name="sessionId">Session ID the driver is using</param>
         /// <param name="name">Name of the command</param>
         /// <param name="parameters">Parameters for that command</param>
+        /// <exception cref="ArgumentNullException">If <paramref name="name"/> is <see langword="null"/>.</exception>
         public Command(SessionId? sessionId, string name, Dictionary<string, object>? parameters)
         {
             this.SessionId = sessionId;
             this.Parameters = parameters ?? new Dictionary<string, object>();
-            this.Name = name;
+            this.Name = name ?? throw new ArgumentNullException(nameof(name));
         }
 
         /// <summary>

dotnet/src/webdriver/CommandInfoRepository.cs

@@ -87,6 +87,7 @@ public bool IsCommandNameDefined(string commandName)
         /// </summary>
         /// <param name="commandName">The <see cref="DriverCommand"/> for which to get the information.</param>
         /// <returns>The <see cref="HttpCommandInfo"/> for the specified command, or <see langword="null"/> if not found or value is not <typeparamref name="T"/>.</returns>
+        /// <exception cref="ArgumentNullException">If <paramref name="commandName"/> is <see langword="null"/>.</exception>
         public T? GetCommandInfo<T>(string commandName) where T : CommandInfo
         {
             T? toReturn = default;