Fix cache not being generated when less than 6 search strings are set

If the user only sets searchString 1, Wordpress will set searchString2-6 to the empty string, instead of NULL. This causes pm_pubmedrequest.php to try to parse [a page](http://www.ncbi.nlm.nih.gov/pubmed/?term=) that do not contain search results. Since it fails, it never writes to pm_cache.xml.

This patch fixes the problem.