CRM-3593 civicrm_event.max_participants should default NULL to match program logic

When you input a form, the form code manually sets max_participants to NULL, however if you use the API and omit this key, this logical default is not used and the field is instead set to 0 (The default in MySQL). To keep things consistent (and to avoid confusion about events that can't have participants) this patch adjusts the default in the table definition.