Fixed Cannot read properties of null (reading 'offsetHeight') errors in markdown card

closes sentry ADMIN-CAV

- the mobile nav element is not always displayed so we should assume we can read it's `offsetHeight` property

ghost/admin/lib/koenig-editor/addon/components/koenig-card-markdown.js

@@ -203,7 +203,7 @@ export default class KoenigCardMarkdown extends Component {
         if (style === fixedBottom) {
             let mobileNav = document.querySelector('.gh-mobile-nav-bar');
 
-            if (mobileNav.offsetHeight) {
+            if (mobileNav?.offsetHeight) {
                 style = `${style}; bottom: ${mobileNav.offsetHeight}px`;
             }
         }