Changing .NET WebDriverBackedSelenium OpenWindow to mimic Java.

The JavaScript evaluated by the .NET WebDriverBackedSelenium OpenWindow
command handler should return a null value from the eval() of the
JavaScript to open the window. This is what the Java implementation does,
and this commit brings the .NET implementation into line with that.

Fixes issue #6966.

Author: jimevans

dotnet/src/webdriverbackedselenium/Internal/SeleniumEmulation/OpenWindow.cs

@@ -31,7 +31,7 @@ public OpenWindow(GetEval windowOpener)
         /// <returns>The result of the command.</returns>
         protected override object HandleSeleneseCommand(IWebDriver driver, string locator, string value)
         {
-            string[] args = { string.Format(CultureInfo.InvariantCulture, "window.open('{0}', '{1}');", locator, value) };
+            string[] args = { string.Format(CultureInfo.InvariantCulture, "window.open('{0}', '{1}'); null;", locator, value) };
 
             this.opener.Apply(driver, args);