When submitting payroll data to the ATO from an STP record without any payslips or employees, a traceback is raised.

Steps to reproduce the error:
- Install ``l10n_au_hr_payroll_account`` module
- Switch to ``My australian Company``
- Go to Payroll > Configuration > Settings > In Australian Localization, Set BMS ID > Set STP Responsible and his date of birth
- Go to Payroll > Reporting > Single Touch Payroll > Create a new record > Set Payment Date > Submit to ATO > Sign & Submit to ATO

Traceback:
```py
IndexError: tuple index out of range
```

https://github.com/odoo/enterprise/blob/b0e48baaf99bdc4faefd2ffdd3bd5637fb548593/l10n_au_hr_payroll_account/models/l10n_au_stp.py#L228-L233
The traceback occurs because ``_get_fiscal_year_start()`` assumes that the
STP record always contains at least one payslip or one employee.
When these recordsets are empty,
indexing the first element raises an IndexError.

Solution:
This commit validates that the required payslips or employees are present
before the submission and raises a validation error instead of a traceback.

Forward-Port-Of: odoo/enterprise#127584
Forward-Port-Of: odoo/enterprise#124096

**Steps to reproduce:**
* Install the **Accounting** module.
* Unhide the **Start Date** and **End Date** fields on invoice lines.
* Create and post a customer invoice with deferred dates.
* Go to **Accounting → Reports → Deferred Revenue Report**.
* Add an annotation to a deferred revenue line by clicking the **annotate** from three dots next to the account.
* Export the report in **XLSX** format.

**Observed behavior:**
* The export fails with a server error: `UnboundLocalError: cannot access local variable 'annotations_x_offset' where it is not associated with a value`

**Cause:**
* The variable `annotations_x_offset` is assigned inside the `for header_level_index, header_level in enumerate(options['column_headers'])` loop, which writes the "Annotations" column header for each header level.
* The Deferred Revenue Report produces an empty `column_headers` list, so the loop body never executes and `annotations_x_offset` is never assigned.
* When the code later tries to write annotation data for each report line, it references the unassigned variable, causing Python to raise `UnboundLocalError`.

**Fix:**
* Introduce a boolean flag `annotations_header_written = False` before the header loop to explicitly track whether the "Annotations" column header has already been written.
* Inside the header loop, set `annotations_header_written = True` after writing the header.
* After writing all individual column headers (where `x_offset` already points to the first free column after all data columns), add a fallback: if `report_annotations` is set but `annotations_header_written` is still `False`, assign `annotations_x_offset` from the current `x_offset` and write the "Annotations" header.

opw-6354473

Forward-Port-Of: odoo/enterprise#122768