FIX Ensure page has extension before calling method

[emteknetnz]

Issue silverstripe/.github#199

Fixes https://github.com/silverstripe/recipe-kitchen-sink/actions/runs/8058976456/job/22012645584?pr=55#step:12:73 on silverstripe/recipe-kitchen-sink#55

1) DNADesign\ElementalUserForms\Tests\ElementFormControllerTest::testElementFormRendering BadMethodCallException: Object->__call(): the method 'getSiteRSSLink' does not exist on 'SilverStripe\UserForms\Control\UserDefinedFormController'

Happened because we removed the cwp versionfeed config which disabled versionfeed.

The elemental userforms tests has the controller extension appied by not the page extension appied, so have just added a little defensive programming here to resolve. It's required for a cms 6 build though may as well fix for 4.13

[GuySartorelli]

Makes sense. I note the commit message suggests you're checking for the extension, but I actually do prefer the way you implemented this (check for the method) which allows developers to add that method themselves in their own extensions.