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 pages that do not contain search results. Since it fails, it never writes to pm_cache.xml.
Merge pull request #1 from ariofrio/patch-1

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