[dotnet] Annotate nullability on WebDriver.SessionId (#15350)

Author: RenderMichael

dotnet/src/webdriver/IHasSessionId.cs

@@ -17,6 +17,8 @@
 // under the License.
 // </copyright>
 
+#nullable enable
+
 namespace OpenQA.Selenium
 {
     /// <summary>

dotnet/src/webdriver/WebDriver.cs

@@ -184,13 +184,13 @@ public ReadOnlyCollection<string> WindowHandles
         /// </summary>
         public bool IsActionExecutor => true;
 
+#nullable enable
+
         /// <summary>
-        /// Gets the <see cref="SessionId"/> for the current session of this driver.
+        /// Gets the <see cref="Selenium.SessionId"/> for the current session of this driver.
         /// </summary>
         public SessionId SessionId { get; private set; }
 
-#nullable enable
-
         /// <summary>
         /// Gets or sets the <see cref="IFileDetector"/> responsible for detecting
         /// sequences of keystrokes representing file paths and names.
@@ -715,8 +715,6 @@ protected bool RegisterInternalDriverCommand(string commandName, [NotNullWhen(tr
             return this.RegisterDriverCommand(commandName, commandInfo, true);
         }
 
-#nullable restore
-
         /// <summary>
         /// Stops the client from running
         /// </summary>
@@ -747,8 +745,6 @@ protected virtual void Dispose(bool disposing)
             this.CommandExecutor.Dispose();
         }
 
-#nullable enable
-
         private static void UnpackAndThrowOnError(Response errorResponse, string commandToExecute)
         {
             // Check the status code of the error, and only handle if not success.