For projects not associated with events, activity creation fails

Discovered when writing unit tests, where "EntityTable1" is supplied as the value for entity_table. Creation of volunteer activity via CRM_Volunteer_BAO_Assignment::createVolunteerActivity() fails with:

CiviCRM_API3_Exception: API (EntityTable1, get) does not exist (join the API team and implement it!)

The method helpfully tries to provide some default values if none are passed, but it does so rather clumsily, assuming that entity_table will always match the pattern civicrm_entity, where entity can be used in an API call:

civicrm_api3(entity, 'getsingle', $params);