Fix issue with empty string value on association key

[iloveitaly]

Normally this would trigger a fatal error. This is related to 34e2965

This revised implementation of the netsuite_extract_from_record function properly handles:

• boolean values. true/false values from netsuite will properly set the AR model
• association fields with empty string. An empty string on an association field will nil-out the association reference
• unset custom fields. If a custom field does not exist on a record, a blank value is used