[TIMOB-17456]: Treat 'null' values as '' for searchableText

testing steps in JIRA
What if searchableText is null and searchText is ""? I think when searchableText is not specified, it means the search functionality is not enabled. No matter you want to show all the items or hide all the items in this case, you can treat this is a special situation and take care of it at the beginning so it does not need to go through all the following logic and code.