When you perform a shop product search on an empty string (or a string consisting of spaces only) an error is written to the log:
2014-03-09 19:55:27,424 ERROR
.magnolia.module.shop.accessors.ShopProductAccesor: Cant read Products
caused by a org.apache.lucene.queryParser.ParseException. See the attached log file for details.
I think the ShopProductSearchResultParagraphModel should check for empty search strings and disallow them.