Insert blank MYSQLTIMESTAMP fields as NULL rather than 0

Propose extending the behaviour for date fields to timestamp fields also. This was hinted at in the comment but maybe there was a reason not to?

Fixes https://lab.civicrm.org/dev/core/-/issues/2558 for me at any rate...

DB/DataObject.php

@@ -1183,9 +1183,9 @@ function insert()
             // CRM-14986 ends
 
             // DATE is empty... on a col. that can be null..
-            // note: this may be usefull for time as well..
+            // Also useful for MYSQLTIMESTAMP to stop NO_ZERO_DATE errors
             if (!$this->$k &&
-                    (($v & DB_DATAOBJECT_DATE) || ($v & DB_DATAOBJECT_TIME)) &&
+                    (($v & DB_DATAOBJECT_DATE) || (($v & DB_DATAOBJECT_TIME) || ($v & DB_DATAOBJECT_MYSQLTIMESTAMP))) &&
                     !($v & DB_DATAOBJECT_NOTNULL)) {
 
                 $rightq .= " NULL ";