correctly checks the driver supports DevTools

When using a driver like EventFiringWebDriver as a parameter for JavaScriptEngine and trying to use StartEventMonitoring the constructor will throw a NullReferenceException. This is caused by casting the driver to IDevTools using "as" which will return null if the casting failed, and incorrectly checking the field "session" instead of "devToolsDriver" for null value.

fixes SeleniumHQ#10776

dotnet/src/webdriver/JavaScriptEngine.cs

@@ -54,7 +54,7 @@ public JavaScriptEngine(IWebDriver driver)
             this.session = new Lazy<DevToolsSession>(() =>
             {
                 IDevTools devToolsDriver = driver as IDevTools;
-                if (session == null)
+                if (devToolsDriver == null)
                 {
                     throw new WebDriverException("Driver must implement IDevTools to use these features");
                 }