fix: use NULL for missing calendar event job order references

[anonymoususer72041]

PR #704 changed optional activity job order references to use NULL instead of the legacy -1 sentinel. Calendar events have the same optional job order relationship, but calendar_event.joborder_id still used -1 for missing job order references.

This updates calendar_event.joborder_id so missing job order references are represented as NULL consistently. The base schema now allows NULL and defaults to NULL and the update migration converts existing -1 values as well as orphaned non-NULL job order references to NULL.

The runtime write paths were updated accordingly. Calendar event creation and updates now pass PHP null when no job order is associated, and Calendar::addEvent() / Calendar::updateEvent() write SQL NULL for that case. Deleting a job order now clears related calendar event job order references by setting them to NULL.

The scope is intentionally limited to calendar_event.joborder_id. Other legacy -1 sentinels, such as calendar_event.data_item_id, calendar_event.data_item_type, activity input sentinels, and filter parameters where -1 has a different meaning, are left unchanged.

[RussH]

Suggest we pin this for the next release, however;

Only thing to check before merge is that modules/install/Schema.php still applies cleanly against current master, as that file often picks up conflicts. Happy to merge.