Merge pull request #198 from eileenmcnaughton/7.x-4.4

CRM-14936. Check for existence of Block module.
Issue CRM-15660. Use CiviCRM custom field handler to render NULL as an empty field.
Merge pull request #200 from xurizaemon/7.x-4.4-CRM-15660

Issue CRM-15660. Use CiviCRM custom handler to render NULL as empty string
CRM-15140 - port further metadata fixes