Saving undefined record fields as NULL in the DB

Before this, undefined record properties would be saved to the DB as the string
`"undefined"' instead of `NULL', which is a bit counter-intuituve; `undefined'
fields now get saved as `NULL' in the DB, and that gets translated back to
`undefined' when those records are fetched from the DB.

Added a unit test for that, plus updated the doc for `sqerl_rec'