Fix bug when converting Vectors containing None (or null) into SQL

[aelred]

isArray always reports false when tokens are in a Vector - this can happen if the original values are in a Vector, and can cause nulls to be removed from the output, e.g.

import org.joda.time.DateTime as JodaDateTime

implicit val optJodaDateTimeArraySetParameter: SetParameter[Seq[Option[JodaDateTime]] = {
  mkArraySetParameter[Option[JodaDateTime]]("timestamptz", _.map(_.toString(myFormatter)).orNull)
}

This converts Vector(None, None) into {,}.

fixes #814

[tminglei]

Merged. Thanks! 👍