Clean up misuse of wfTimestampOrNull()

Clean up misuse of wfTimestampOrNull()

It's primarily meant for nullable timestamps in the database.
And for the mere purpose of an is_null() check, it's more confusion
than useful to cause this function since in these two cases its return
value is not used as-is but further conditionalized.

* Also fix fragile link between getRegistration()'s false return
  value never being passed to wfTimestamp (to mean "now") because
  of the isLoggedIn() guard. The ternary would now make those
  follow the 'else' branch toward null.

Change-Id: I9a1ee2a56e1767bfb750e27b1f37bdaeb5e6378d