Do not search for elements if no previews have been registered

The findNearestEligibleTarget method is now being called with an
empty array and thus an empty string as a selector

Follow up to
I5f293a134521f086c9f62babb9d06cd9c51d7d47

Bug: T355933
Bug: T356186
Bug: T356193
Change-Id: I3af44ae65d5097fc44744838a4edb07552568b17
(cherry picked from commit 51059828bf)

src/preview/model.js

@@ -123,8 +123,11 @@ const elementMatchesSelector = ( element, selector ) => {
  * @return {HTMLElement|null}
  */
 export function findNearestEligibleTarget( element ) {
-	const selector = selectors.join( ', ' );
-	return element.closest( selector );
+	if ( selectors.length ) {
+		const selector = selectors.join( ', ' );
+		return element.closest( selector );
+	}
+	return null;
 }
 
 /**