[dotnet] do not execute Quit command when sessionId is not available

If new session creation fails a Quit() method is called which calls Dispose and then Quit command, unfortunately in this case
session is still not created and it throws null reference exception and this hides the actual exception

Author: Indomitable

dotnet/src/webdriver/WebDriver.cs

@@ -703,7 +703,10 @@ protected virtual void Dispose(bool disposing)
         {
             try
             {
-                this.Execute(DriverCommand.Quit, null);
+                if (this.sessionId is not null)
+                {
+                    this.Execute(DriverCommand.Quit, null);
+                }
             }
             catch (NotImplementedException)
             {