Scalar subqueries should work with NULL return value

[Finn Völkel]

  (xt/submit-tx node [[:put-docs :foo {:xt/id 1}]])

  (xt/q node '(-> (rel [{}] [])
                  (with {:some-data (q (from :foo [xt/id]))})))
  ;; => [{:some-data 1}]

  (xt/q node '(-> (rel [{}] [])
                  (with {:some-data (q (rel [{}] [a]))})))
  ;; => Execution error (NullPointerException) at xtdb.types/field->col-type (types.clj:474).
  ;;    Cannot invoke "org.apache.arrow.vector.types.pojo.Field.getType()" because "field" is null