Fix bug in parsing of SnglInspiral tables with omitted columns

The bug was due to the incorrect assumption about the default value
of nullable Django fields (`Field(null=True)`). The assumption was
that `field.default` is `None`, but in fact `field.default` has the
sentinel value of `NOT_PROVIDED`.
2 jobs for parse-coinc-xml-omitted-columns in 20 minutes and 43 seconds (queued for 1 second)