Add checks to glob calls for file exists to avoid PHP Warning

Add checks to glob calls for file exists to avoid PHP Warning.
If there are no '*_install.sql' files right now we get a warning like:
Warning: Invalid argument supplied for foreach() in CRM_Pcpteams_Upgrader_Base->onInstall() (line 244 of /var/www/myhost/sites/all/civicrm_custom/extensions/ca.bidon.pcpteams/CRM/Pcpteams/Upgrader/Base.php).
Stray brackets
Merge pull request #21 from lolas-freeform/patch-1

A patch to avoid a warning if an upgrader was added but no '*_install.sql' files exist in extension folder.