Return empty from Backend::dateTime instead of current date (#3246)

This changes Backend::dateTime to return an empty string instead of the current date when dealing with a null date. It should only affect the date field preview instance. Credit to @pvullioud

modules/backend/helpers/Backend.php

@@ -118,6 +118,10 @@ class Backend
             'timeSince' => false,
             'ignoreTimezone' => false,
         ], $options));
+        
+        if(!$dateTime) {
+            return '';
+        }
 
         $carbon = DateTimeHelper::makeCarbon($dateTime);