### Steps to Reproduce:
1. Go to Helpdesk and create an SLA
2. Create a new Ticket
3. Use Studio to add the field `sla_reached`
4. Apply the SLA
5. Notice that the field is True

### Description of the issue/feature this PR addresses:
**Issue:** The `_compute_sla_reached` method in on `helpdesk_ticket.py` determines whether an SLA has been reached by checking if `exceeded_hours` is less than 0 on its `helpdesk.sla.status` records. Since every newly assigned SLA has a future deadline, `exceeded_hours` starts as negative (AKA <0), so the ticket is immediately flagged as `sla_reached = True` before any progress has been made. Conversely, a ticket that reaches its target stage after the deadline has `exceeded_hours >= 0`, so it's incorrectly flagged as `sla_reached = False` even though the SLA target was genuinely reached (it was just late).

**Solution:** Change the domain in `_compute_sla_reached` to check `reached_datetime != False` instead of `exceeded_hours < 0`, matching the field already listed in the method's `@api.depends` and the same field `_sla_reach()` sets when a ticket enters its target stage. This makes `sla_reached` reflect actual stage progression rather than a time-remaining calculation.

### Current behavior before PR: 
A newly created ticket with a pending SLA shows `sla_reached = True` and `sla_success = True` immediately upon creation, before the ticket has moved to any target stage. SImilarly, a ticket that reaches its target stage after the SLA deadline is incorrectly marked `sla_reached = False`.

### Desired behavior after PR: 
`sla_reached` is False on ticket creation and only becomes True once the ticket actually enters the SLA's target stage. Lateness continues to be tracked separately and correctly via `sla_reached_late`

opw-6361851

The fix introduced in https://github.com/odoo/enterprise/commit/9e9c99712ad4b9d58dc7601da41a852e457ad097 didn't account for the fact that `post.get('media_url') ` could return a None value, which in turn would raise en error when trying to concatenate the value later.

This in turn:
- will block syncing of instagram instagram posts
- will raise a traceback when you open the Social Marketing module and the auto-sync kicks in.
### Example traceback
```
Traceback (most recent call last):
[...]
  File "/home/odoo/src/enterprise/saas-19.2/social_instagram/models/social_stream.py", line 86, in _fetch_stream_data
    return self._fetch_instagram_posts()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/odoo/src/enterprise/saas-19.2/social_instagram/models/social_stream.py", line 64, in _fetch_instagram_posts
    values['message'] = (values['message'] + "\n" + post.get('media_url')).strip()
                         ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
TypeError: can only concatenate str (not "NoneType") to str
```

### Solution:
Fallback to an empty string if `post.get('media_url')` yields a None value.

OPW-6449357

Forward-Port-Of: odoo/enterprise#126987