[NO ISSUE][FUN] DATETIME constructor should accept TIME

[NO ISSUE][FUN] DATETIME constructor should accept TIME

- user model changes: no
- storage format changes: no
- interface changes: no

Details:
- Fix datetime() constructor function to handle TIME
  argument, instead of returning NULL

Change-Id: I9eab6b3e29642ef6d7f88808e7eb446e8f7c455c
Reviewed-on: https://asterix-gerrit.ics.uci.edu/c/asterixdb/+/15203
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Integration-Tests: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Reviewed-by: Dmitry Lychagin <dmitry.lychagin@couchbase.com>
Reviewed-by: Ian Maxon <imaxon@uci.edu>