Add defensive coding to the luceneapi_query_parse() API function

The fix for #711394: Search Lucene displays every item ignores search keyword added logic that detects when terms were passed by the user but none met the minimum length requirement set in the Search Lucene API administrative page. As illustrated in the issue #718834: white page when used with luceneapi 6.x.2.1, the fix has the side effect of throwing fatal errors when an empty string is passed as the search keys to this function. Although an empty search should get detected before this function is called, defensive coding should be added to handle empty strings gracefully.